derivations: Do not fetch narinfos for non-substitutable items.

This avoids connections to substitute servers for derivations that are
not substitutable anyway, such as profiles.

Reported by Andy Wingo.

* guix/derivations.scm (substitution-oracle): Skip derivations that do
not pass 'substitutable-derivation?'.
* tests/derivations.scm ("substitution-oracle and #:substitute? #f"):
New test.

gnu: epiphany: Update to 3.22.7.

* gnu/packages/gnome.scm (epiphany): Update to 3.22.7.

gnu: webkitgtk: Update to 2.16.0.

WebKitGTK+ 2.16.0 is the first stable release of WebKitGTK+ that uses libgcrypt
instead of GnuTLS for some cryptographic functions:

<https://bugs.webkit.org/show_bug.cgi?id=163125>
<http://trac.webkit.org/changeset/208297/webkit>

* gnu/packages/webkit.scm (webkitgtk): Update to 2.16.0.
[inputs]: Replace gnutls with libgcrypt.

gnu: grafx2: Remove zero-width space.

* gnu/packages/game-development.scm (grafx2)[description]: Remove
invisible <U+200B> (‘zero-width space’) character.

gnu: libgit2: Patch use-after-free.

* gnu/packages/patches/libgit2-use-after-free.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add it.
* gnu/packages/version-control.scm (libgit2)[inputs]: Add it.
[arguments] <#:phases>: Add 'apply-patch' phase.

gnu: lm-sensors: Use a descriptive file name for the source tarball.

* gnu/packages/linux.scm (lm-sensors)[source]: Add file-name field.

gnu: mcelog: Update to 149.

* gnu/packages/linux.scm (mcelog): Update to 149.

gnu: corkscrew: Install the README file.

* gnu/packages/ssh.scm (corkscrew)[arguments]: Add a very basic
‘install-documentation’ phase.

gnu: haunt: Disable test suite.

* gnu/packages/guile.scm (haunt): Disable test suite.

gnu: Add grafx2.

* gnu/packages/game-development.scm (grafx2): New variable.

gnu: geoclue: Update to 2.4.6.

* gnu/packages/gnome.scm (geoclue): Update to 2.4.6.

gnu: corkscrew: Improve descriptions.

* gnu/packages/ssh.scm (corkscrew)[synopsis, description]: Fix typos and
condense.  Remove scary warning (the tunnel is always encrypted through
SSH) and speculation.

gnu: corkscrew: Download source from a mirror.

* gnu/packages/ssh.scm (corkscrew)[source]: Use
‘https://downloads.openwrt.org’.

gnu: mosh: Update to 1.3.0.

* gnu/packages/ssh.scm (mosh): Update to 1.3.0.

gnu: youtube-dl: Update to 2017.03.26.

* gnu/packages/video.scm (youtube-dl): Update to 2017.03.26.

gnu: gnuplot: Enable tikz backend.

* gnu/packages/maths.scm (gnuplot)[inputs]: Add lua.

ui: Support Texinfo markup in package synopses.

* guix/ui.scm (package-field-string): New procedure.
(package-description-string): Use it.
(package-synopsis-string): New procedure.
(package->recutils): Use it.
* guix/scripts/lint.scm (check-synopsis-style)[check-texinfo-markup]:
New procedure.  Use it in checks.
* tests/lint.scm: Test it.
* gnu/packages/perl.scm (perl-try-tiny)[synopsis]: Adjust for the
Texinfo markup.

maint: Give command to install the pre-push hook.

* HACKING: Add 'cp' line to install the pre-push hook.

Revert "gnu: audacity: Use gtk+@3."

It appears that Audacity does not yet support GTK+-3 fully; the user interface
was unusable with this change.

This reverts commit 36e17ab417078710939771d5ab19976eec257309.

gnu: claws-mail: Update to 3.15.0.

* gnu/packages/mail.scm (claws-mail): Update to 3.15.0.