substitute-binary: Work around thread-unsafe `regexp-exec'.

* guix/scripts/substitute-binary.scm (%regexp-exec-mutex): New variable.
  (string->uri): New procedure.
  (fields->alist): Wrap `regexp-exec' call in `with-mutex'.

Augment `.dir-locals.el'.

* .dir-locals.el: Add more Scheme settings.
* guix-build.in, tests/base32.scm, tests/build-utils.scm,
  tests/builders.scm, tests/derivations.scm, tests/packages.scm,
  tests/snix.scm, tests/store.scm, tests/union.scm, tests/utils.scm:
  Remove redundant Emacs local variable settings.

.dir-locals.el: Fill at 78 columns.

Add (guix ui).

* guix/ui.scm: New file.
* Makefile.am (MODULES): Add it.
* po/POTFILES.in: Add it.

* guix-build.in: Use it.
  (_, N_, leave): Remove.
  (guix-build): Use `with-error-handling' instead of the `guard' form.
* guix-download.in: Use it.
  (_, N_, leave): Remove.

distro: Add the `guile', `typesetting', and `databases' package modules.

* distro.scm (package-files): Adjust PREFIX-LEN to use the top-level
  directory, not %DISTRO-MODULE-DIRECTORY.
* distro/packages/base.scm (guile-reader, guile-reader/guile-1.8,
  guile-reader/guile-2.0, lout, recutils): Move to...
* distro/packages/guile.scm, distro/packages/typesetting.scm,
  distro/packages/databases.scm: ... here.  New files.
* Makefile.am (MODULES): Add them.
  (EXTRA_DIST): Add `.dir-locals.el'.

* .dir-locals.el: New file, with settings formerly in `base.scm'.