offload: Allow testing machines that match a regexp.

* guix/scripts/offload.scm (check-machine-availability): Add 'pred'
parameter and honor it.
(guix-offload): for the "test" sub-command, accept an extra 'regexp'
parameter.  Pass a second argument to 'check-machine-availability'.

doc: Correct example for 'wpa-supplicant-service-type'.

* doc/guix.texi (Networking Services): Change example to use
'wpa-supplicant-service-type'.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

offload: Make the compression methods configurable.

* guix/scripts/offload.scm (<build-machine>)[compression]
[compression-level]: New fields.
(open-ssh-session): Honor them.
* doc/guix.texi (Daemon Offload Setup): Document them.

guix system: If the new system generation already exists, overwrite it.

Fixes <http://bugs.gnu.org/25100>.

Before that, 'guix system reconfigure' would fail if the next generation
already existed.

* guix/scripts/system.scm (switch-to-system): Use 'switch-symlink'
instead of 'symlink'.
* doc/guix.texi (Using the Configuration System, Invoking guix system):
Document the behavior.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

offload: Add "test" sub-command.

* guix/scripts/offload.scm (assert-node-repl, assert-node-has-guix)
(nonce, assert-node-can-import, assert-node-can-export)
(check-machine-availability): New procedures.
(%random-state): New variable.
(guix-offload): Add case for "test".
* doc/guix.texi (Daemon Offload Setup): Document it.  Remove obsolete
bit about remote invocation of 'guix build'.

doc: Use @file to mark file names.

* doc/guix.texi: Use @file where appropriate.

gnu: Add Kerberos client service.

* doc/guix.texi (Kerberos Services)[Krb5 Service]: New subsubheading.
* gnu/services/kerberos.scm (krb5-service-type): New variable.
* gnu/services/configuration.scm (configuration-field-serializer,
  configuration-field-getter): Export variables.

doc: Remove obsolete bit about networking in VMs.

* doc/guix.texi (Running GuixSD in a VM): Remove "the boot will fail"
wording since that is no longer true since commit
6129dd8b5989f77b2976c68ecdf1f7dbfa63ec46.

refresh: '--list-updaters' shows updater coverage.

* guix/scripts/refresh.scm (list-updaters-and-exit): Compute the
coverage ratio of each updater and print it.  Print the coverage ratio
for all the updaters.
* doc/guix.texi (Invoking guix refresh): Document it.

gnu-maintenance: Add kernel.org updater.

* guix/gnu-maintenance.scm (latest-kernel.org-release): New procedure.
(%kernel.org-updater): New variable.
* guix/scripts/refresh.scm (%updaters): Add it.

services: Add 'cuirass-service'.

* gnu/services/cuirass.scm: New file.
* gnu/local.mk (GNU_SYSTEM_MODULES): Add it.
* doc/guix.texi (Continuous integration): New node.

Merge branch 'python-build-system'.

Merge branch 'master' into python-build-system

refresh: Warn about packages that lack an updater.

* guix/upstream.scm (package-update-path): Rename to...
(package-latest-release): ... this.  Remove 'version>?' check.
(package-latest-release*): New procedure.
(package-update): Use it.
* guix/scripts/refresh.scm (lookup-updater): Rename to...
(lookup-updater-by-name): ... this.
(warn-no-updater): New procedure.
(update-package): Add #:warn? parameter and honor it.
(check-for-package-update): New procedure.
(guix-refresh)[warn?]: New variable.
Replace inline code when UPDATE? is false with a call to
'check-for-package-update'.
Pass WARN? to 'check-for-package-update' and 'update-package'.
* doc/guix.texi (Invoking guix refresh): Document it.  Fix a couple of
typos.

doc: Document NSS incompatibility issues on foreign distros.

* doc/guix.texi (Application Setup)[Name Service Switch]: New
subsection.

doc: mysql-configuration: Fix typo.

* doc/guix.texi (Database Services): Fix typo of 'mysql-configuration'.

services: Add opensmtpd service.

* gnu/services/mail.scm (<opensmtpd-configuration>): New record type.
(%default-opensmtpd-config-file, %opensmtpd-accounts): New variables.
(opensmtpd-shepherd-service, opensmtpd-activation): New procedures.
(opensmtpd-service-type): New variable.
* doc/guix.texi (Mail Services): Document it.

offload: Rewrite to make direct RPCs to the remote daemon.

* guix/scripts/offload.scm (<build-machine>)[daemon-socket]: New field.
(connect-to-remote-daemon): New procedure.
(%gc-root-file, register-gc-root, remove-gc-roots, offload): Remove.
(transfer-and-offload): Rewrite using 'connect-to-remote-daemon' and
RPCs over SSH.
(store-import-channel, store-export-channel): New procedures.
(send-files, retrieve-files): Rewrite using these.

offload: Use Guile-SSH instead of GNU lsh.

* guix/scripts/offload.scm (<build-machine>)[ssh-options]: Remove.
[host-key, host-key-type]: New fields.
(%lsh-command, %lshg-command, user-lsh-private-key): Remove.
(user-openssh-private-key, private-key-from-file*): New procedures.
(host-key->type+key, open-ssh-session): New procedures.
(remote-pipe): Remove 'mode' parameter.  Rewrite in terms of
'open-ssh-session' etc.  Update users.
(send-files)[missing-files]: Rewrite using the bidirectional channel
port.
Remove call to 'call-with-compressed-output-port'.
(retrieve-files): Remove call to 'call-with-decompressed-port'.
(machine-load): Remove exit status logic.
* doc/guix.texi (Requirements): Mention Guile-SSH.
(Daemon Offload Setup): Document 'host-key' and 'private-key'.  Show the
default value on each @item line.
* m4/guix.m4 (GUIX_CHECK_GUILE_SSH): New macro.
* config-daemon.ac: Use 'GUIX_CHECK_GUILE_SSH'.  Set
'HAVE_DAEMON_OFFLOAD_HOOK' as a function of that.

Merge branch 'master' into python-build-system