Merge branch 'master' into core-updates

Conflicts:
	Makefile.am
	gnu/packages/autotools.scm
	gnu/packages/guile.scm
	gnu/packages/python.scm
	gnu/packages/shishi.scm
	guix/gnu-maintenance.scm
	guix/scripts/build.scm
	guix/scripts/gc.scm
	guix/scripts/package.scm
	guix/scripts/substitute-binary.scm
	guix/ui.scm
	nix/nix-daemon/guix-daemon.cc
	test-env.in
	tests/nar.scm
	tests/store.scm

packages: Use `read' and source properties for `package-field-location'.

* guix/packages.scm (package-field-location): Rewrite using `read' and
  source properties.  Change to return #f upon failure.
* tests/packages.scm ("package-field-location"): Check for #f upon failure.
* build-aux/sync-synopses.scm: Adjust accordingly.

packages: Add `package-field-location'.

* guix/packages.scm (package-field-location): New procedure.
* build-aux/sync-synopses.scm: Use it instead of `package-location'.
* tests/packages.scm ("package-field-location"): New test.

build: Add `sync-synopses.scm'.

* build-aux/sync-synopses.scm: New file.
* Makefile.am (EXTRA_DIST): Add it.
  (dist-hook): New target.