substitute: Use ~/.cache when invoked by an unprivileged user.

This is a followup to ea0c6e0507a6997f12a4f29d0445b51cf53bd81e.

* guix/scripts/substitute.scm (%narinfo-cache-directory): Use
'cache-directory' when (getuid) returns non-zero.
(cache-narinfo!): Remove 'catch'.

substitute: Internationalize the "Downloading" message.

* guix/scripts/substitute.scm (process-substitution): I18n "Downloading"
message.

substitute: Gracefully handle invalid store file names.

Before, something like:

  echo have /gnu/foo | ./test-env guix substitute --query

would lead to an ugly backtrace.

* guix/scripts/substitute.scm (narinfo-cache-file): Call 'leave' when
'store-hash-part' returns #f.

Merge branch 'master' into gnome-updates

utils: Move combinators to (guix combinators).

* guix/utils.scm (compile-time-value, memoize, fold2)
(fold-tree, fold-tree-leaves): Move to...
* guix/combinators: ... here.  New file.
* tests/utils.scm ("fold2, 1 list", "fold2, 2 lists")
(fold-tree tests): Move to...
* tests/combinators.scm: ... here.  New file.
* Makefile.am (MODULES, SCM_TESTS): Add them.
* gnu/packages.scm, gnu/packages/bootstrap.scm,
gnu/services/herd.scm, guix/build-system/gnu.scm,
guix/build-system/python.scm, guix/derivations.scm,
guix/gnu-maintenance.scm, guix/import/elpa.scm,
guix/scripts/archive.scm, guix/scripts/build.scm,
guix/scripts/graph.scm, guix/scripts/lint.scm,
guix/scripts/size.scm, guix/scripts/substitute.scm,
guix/serialization.scm, guix/store.scm, guix/ui.scm: Adjust imports
accordingly.

substitute: Sanitize the client-provided column number.

* guix/scripts/substitute.scm (client-terminal-columns): Sanitize the
client's column number.

substitute: Install the client's locale.

* guix/store.scm (set-build-options): Add #:locale parameter and honor
it.
* guix/scripts/substitute.scm (guix-substitute): Install the client's
locale.

substitute: Internationalize signature and download messages.

* guix/scripts/substitute.scm (assert-valid-narinfo): Use gettext for
messages.

substitute: Better abbreviate substitute URL in progress report.

Suggested by Danny Milosavljevic <dannym@scratchpost.org>.

* guix/build/download.scm (nar-uri-abbreviation): New procedure.
* guix/scripts/substitute.scm (process-substitution): Use it instead of
'store-path-abbreviation'.

substitute: Honor the number of columns of the client terminal.

* guix/store.scm (set-build-options): Add #:terminal-columns parameter
and honor it.
* guix/scripts/substitute.scm (client-terminal-columns): New procedure.
(guix-substitute): Use it to parameterize 'current-terminal-columns'.

substitute: Gracefully handle TLS errors.

* guix/scripts/substitute.scm (with-networking): Use 'match-lambda*' and
add case for 'gnutls-error'.

substitute: Do not leak file descriptors for TLS connections.

Partially fixes <http://bugs.gnu.org/20145>.

* guix/scripts/substitute.scm (fetch, download-cache-info):
(http-multiple-get, fetch-narinfos, progress-report-port): Use
'close-connection' instead of 'close-port'.

substitute: Cache transient HTTP errors for 10mn.

* guix/scripts/substitute.scm (fetch-narinfos)[handle-narinfo-response]:
Cache transient errors for 10mn.
(%narinfo-transient-error-ttl): New variable.

substitute: Update progress for responses different from 200/404.

* guix/scripts/substitute.scm (fetch-narinfos)[handle-narinfo-response]:
Add missing call to 'update-progress!'.

substitute: Honor client-provided empty URL list.

Before that, 'guix build --substitute-urls=""' would lead to using the
daemon's own URL list instead of the empty list.  The 'or*' hack, which
is to blame, had become unnecessary since commit
fb4bf72be3fbc23bca35ba4b842b7e1517ef0e3a.

Reported by Mark H Weaver <mhw@netris.org>.

* guix/scripts/substitute.scm (or*): Remove.
(%cache-urls): Use 'or' instead of 'or*'.
* tests/store.scm ("substitute query, alternating URLs"): Add test with
empty URL list.
* doc/guix.texi (Common Build Options): Mention the empty string.

substitute: Honor the 'max-age' of 'Cache-Control' headers.

This allows substitute servers to tell 'guix substitute' how long they
can cache narinfo lookups.

* guix/scripts/substitute.scm (cache-narinfo!): Add 'ttl' parameter.
[cache-entry]: Honor it.
(fetch-narinfos)[handle-narinfo-response]: Check the 'Cache-Control'
header of RESPONSE and pass its 'max-age' value to 'cache-narinfo!'.

substitute: Make room for a 'ttl' field in cached entries.

* guix/scripts/substitute.scm (cached-narinfo): Expect 'narinfo' sexp
version 2 with a 'ttl' field.
(cache-narinfo!)[cache-entry]: Produce 'narinfo' sexp version 2 with a
'ttl' field.
(remove-expired-cached-narinfos)[expired?]: Read 'narinfo' sexp version 2.

Merge branch 'master' into security-updates

substitute: Keep the initial connection alive.

The connection used to fetch /nix-cache-info is now reused for the
subsequent narinfo requests.

* guix/scripts/substitute.scm (download-cache-info)[download]: Remove.
[uri, read-cache-info]: New variables.
Rewrite in terms of 'http-fetch' instead of 'fetch'.  Return an open
port in addition to a <cache-info>.
* guix/scripts/substitute.scm (http-multiple-get): Add #:port parameter
and honor it.
(fetch-narinfos)[do-fetch]: Add 'port' parameter.
Adjust to new 'download-cache-info' and 'do-fetch' signatures.

substitute: Remove dead code.

This parameter became unused with the switch to HTTP pipelining in
commit d3a652037ef879f9279bc056c43d15ba7afcbb25.

* guix/scripts/substitute.scm (fetch): Remove #:quiet-404? and adjust
accordingly.