import: cpan: Add updater.

* guix/import/cpan.scm (module->dist-name): Fetch the field of interest.
(cpan-fetch): Accept release name rather than module name.
(fix-source-url): Rename to ...
(cpan-source-url): ... this.  Take metadata as parameter.
(package->upstream-name, cpan-version, cpan-package?, latest-release):
New procedures.
(cpan-module->sexp): Use cpan-version and cpan-source-url.
(%cpan-updater): New variable.
* guix/scripts/refresh.scm (%updaters): Add %CPAN-UPDATER.

Merge branch 'master' into staging

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'.

offload: Test each machine only once.

* guix/scripts/offload.scm (check-machine-availability)[build-machine=?]:
New procedure.
Add call to 'delete-duplicates'.

offload: Do not read ~/.ssh/known_hosts.

* guix/scripts/offload.scm (open-ssh-session): Pass #:knownhosts to
'make-session'.

Merge remote-tracking branch 'origin/master' into staging

offload: Increase the connection timeout.

* guix/scripts/offload.scm (open-ssh-session): Set #:timeout to 10.

offload: Send the build log to the right file descriptor.

This fixes a regression introduced in
21531add3205e400707c8fbfd841845f9a71863a whereby the build log would no
longer be sent to FD 4, thereby leading the daemon to not see the build
log.

* guix/scripts/offload.scm (transfer-and-offload): Parameterize
CURRENT-BUILD-OUTPUT-PORT.

offload: Fix plural of some messages.

* guix/scripts/offload.scm (send-files): Use 'N_' for possibly plural
message.  Write "store item" instead of "store file".
(retrieve-files): Likewise.

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>

refresh: Indicate that PACKAGE... is optional.

* guix/scripts/refresh.scm (show-help): Add brackets around PACKAGE.

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'.

publish: Factorize 'content-length' addition.

* guix/scripts/publish.scm (with-content-length): New procedure.
(http-write) <application/octet-stream>: Use it.

Merge branch 'master' into staging

offload: Do not abort when a machine is unreachable.

* guix/scripts/offload.scm (machine-load): Wrap 'open-ssh-session' call
in 'false-if-exception'; return +inf.0 if it returns #f.

offload: Gracefully report connection failures.

* guix/scripts/offload.scm (open-ssh-session): Check the return value of
'connect!'.  Call 'leave' when it's not 'ok.

offload: Warn about the lack of zlib support.

* guix/scripts/offload.scm (guix-offload): Print a warning when
'zlib-support?' returns false.

offload: Remove redundant call to 'topologically-sorted' in 'send-file'.

* guix/scripts/offload.scm (send-files): Remove call to
'topologically-sorted'.

Merge branch 'master' into staging