distro: Use (guix licenses) instead of strings.

* distro/packages/acl.scm, distro/packages/attr.scm,
  distro/packages/autotools.scm, distro/packages/base.scm,
  distro/packages/bash.scm, distro/packages/bdw-gc.scm,
  distro/packages/bison.scm, distro/packages/bootstrap.scm,
  distro/packages/compression.scm, distro/packages/cpio.scm,
  distro/packages/ddrescue.scm, distro/packages/ed.scm,
  distro/packages/flex.scm, distro/packages/gawk.scm,
  distro/packages/gdbm.scm, distro/packages/gettext.scm,
  distro/packages/gnupg.scm, distro/packages/gnutls.scm,
  distro/packages/gperf.scm, distro/packages/guile.scm,
  distro/packages/help2man.scm, distro/packages/less.scm,
  distro/packages/libffi.scm, distro/packages/libsigsegv.scm,
  distro/packages/libunistring.scm, distro/packages/linux.scm,
  distro/packages/lout.scm, distro/packages/lsh.scm,
  distro/packages/m4.scm, distro/packages/multiprecision.scm,
  distro/packages/nano.scm, distro/packages/ncurses.scm,
  distro/packages/nettle.scm, distro/packages/perl.scm,
  distro/packages/pkg-config.scm, distro/packages/pth.scm,
  distro/packages/readline.scm, distro/packages/recutils.scm,
  distro/packages/shishi.scm, distro/packages/system.scm,
  distro/packages/texinfo.scm, distro/packages/time.scm,
  distro/packages/wget.scm, distro/packages/which.scm,
  distro/packages/zile.scm: Use (guix licenses).

distro: Use `mirror://' URLs.

* distro/packages/base.scm, distro/packages/bash.scm,
  distro/packages/compression.scm, distro/packages/gawk.scm,
  distro/packages/gperf.scm, distro/packages/guile.scm,
  distro/packages/libsigsegv.scm, distro/packages/libtool.scm,
  distro/packages/libunistring.scm, distro/packages/m4.scm,
  distro/packages/multiprecision.scm, distro/packages/ncurses.scm,
  distro/packages/pth.scm, distro/packages/readline.scm,
  distro/packages/recutils.scm: Use `mirror://gnu' URLs.
* distro/packages/gnupg.scm: Use `mirror://gnupg' URLs.

Remove (guix http) and (guix ftp).

* guix/ftp.scm, guix/http.scm, guix/build/ftp.scm, guix/build/http.scm:
  Remove.
* Makefile.am (MODULES): Likewise.
* tests/builders.scm, distro/packages/base.scm,
  distro/packages/bash.scm, distro/packages/bdw-gc.scm,
  distro/packages/compression.scm, distro/packages/gawk.scm,
  distro/packages/gnupg.scm, distro/packages/gperf.scm,
  distro/packages/guile.scm, distro/packages/libffi.scm,
  distro/packages/libsigsegv.scm, distro/packages/libtool.scm,
  distro/packages/libunistring.scm, distro/packages/lout.scm,
  distro/packages/m4.scm, distro/packages/multiprecision.scm,
  distro/packages/ncurses.scm, distro/packages/perl.scm,
  distro/packages/pkg-config.scm, distro/packages/pth.scm,
  distro/packages/readline.scm, distro/packages/recutils.scm: Use
  `url-fetch' instead of `http-fetch' and `ftp-fetch'.
* distro/packages/bootstrap.scm: Likewise
  (bootstrap-origin): Remove references to `http-fetch' and
  `ftp-fetch'.
* guix.scm (%public-modules): Remove `http' and `ftp'; add `download'.

distro: recutils: Comment out optional dependencies.

* distro/packages/recutils.scm (recutils): Comment out optional
  dependencies not packaged here.

distro: Go for one module per package.

The alternative, which was to use one module per category, would
probably not scale well.  First, because many packages could fall into
several categories (does GnuTLS go into "networking", "security", or
"libraries"?).  Second, because that could easily lead to circular
dependencies among modules ("security" and "networking" depend on each
other, etc.)

* distro/packages/databases.scm: Rename to...
* distro/packages/recutils.scm: ... this.
* distro/packages/typesetting.scm: Rename to...
* distro/packages/lout.scm: ... this.
* Makefile.am (MODULES): Adjust accordingly.
* po/POTFILES.in: Likewise.

packages: `description' → `synopsis', `long-description' → `description'.

* guix/packages.scm (<package>): Rename `description' to `synopsis', and
  `long-description' to `description'.
* tests/packages.scm, distro/packages/base.scm,
  distro/packages/databases.scm, distro/packages/guile.scm,
  distro/packages/typesetting.scm:  Update accordingly.
* po/Makevars (XGETTEXT_OPTIONS): Update `--keyword' flags accordingly.

build: Clearly mark Nixpkgs as optional.

* configure.ac: Always show the result of checking for Nixpkgs.  Don't
  warn when Nixpkgs is not found.
* Makefile.am (AM_DISTCHECK_CONFIGURE_FLAGS): Remove `--with-nixpkgs'
  flag.
* guix/utils.scm (%nixpkgs-directory): Use either the compile-time or
  the run-time `NIXPKGS' environment variable.
* release.nix (jobs.tarball, jobs.build): Remove `--with-nixpkgs'
  configure flag.
* README: Mark Nixpkgs as optional.

* distro/packages/databases.scm, distro/packages/guile.scm,
  distro/packages/typesetting.scm: Change uses of `nixpkgs-derivation*'
  to `nixpkgs-derivation', to avoid failing at compile-time.

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'.