guix-package: Allow `--roll-back' to skip missing generations.

* guix-package.in (profile-numbers): New procedure.
  (latest-profile-number): Use it.
  (previous-profile-number): New procedure.
  (roll-back): Use it lieu of `1-'.  Check whether PREVIOUS-NUMBER is
  zero, and raise an error when it is.
* tests/guix-package.sh: Test whether we can roll back over a "hole".

guix-package: Add `--roll-back'.

Based on a patch by Nikita Karetnikov <nikita@karetnikov.org>.

* guix-package.in (profile-regexp): New procedure.
  (latest-profile-number): Remove `%profile-rx', and use
  `profile-regexp' instead.
  (profile-number, roll-back): New procedure.
  (show-help): Add `--roll-back'.
  (%options): Likewise.
  (guix-package)[process-actions]: First check whether `roll-back?' is
  among OPTS, and call `roll-back' if it is, followed by a recursive
  call to `process-actions'.  Emit the "nothing to be done" message only
  when INSTALL or REMOVE is non-empty.
* tests/guix-package.sh (readlink_base): New function.
  Add tests for `--roll-back'.
* doc/guix.texi (Invoking guix-package): Document `--roll-back'.

distro: gnutls: Update to 3.1.6.

* distro/packages/gnutls.scm (gnutls): Update to 3.1.6.

packages: Change `base32' to allow non-literal strings.

* guix/packages.scm (base32): Allow arguments that are not literal strings.

distro: nettle: Update to 2.6.

* distro/packages/nettle.scm (nettle): Update to 2.6.

distro: gdb: Add dependency on Expat and Python.

* distro/packages/gdb.scm (gdb): Add Expat and Python as inputs.

distro: Add Expat.

* distro/packages/xml.scm: New file.
* Makefile.am (MODULES): Add it.

doc: Start documenting derivations.

* doc/guix.texi (Defining Packages): Add cross-reference to
  "Derivations".
  (The Store): Add `build-derivations'.
  (Derivations): Populate.

doc: Fix typos, and improve daemon documentation.

* doc/guix.texi: Fix typos.
  (Invoking guix-daemon): Add details about `--disable-log-compression'
  and `--disable-store-optimization'.
  Reported by Nikita Karetnikov <nikita@karetnikov.org>.

doc: Remove @documentlanguage.

* doc/guix.texi: Remove @documentlanguage, which confuses TeX.

distro: Add icu4c (International Components for Unicode).

* distro/packages/icu4c.scm: New file.
* Makefile.am (MODULES): Add it.

distro: Add Python 2.7.3.

* distro/packages/python.scm: New file.
* Makefile.am (MODULES): Add it.

doc: Start documenting (guix store).

* doc/guix.texi (The Store): Populate.
  (Introduction): Add cross-reference.  Change "package store" to "the
  store".

distro: recutils: Add 3 optional dependencies.

* distro/packages/recutils.scm (recutils): Add Emacs, Check, and bc as
  dependencies.

distro: check: Fix source URL.

* distro/packages/check.scm (check): Fix source URI.

distro: Add patches forgotten in previous commits.

* distro/packages/patches/bigloo-gc-shebangs.patch,
  distro/packages/patches/emacs-configure-sh.patch,
  distro/packages/patches/procps-make-3.82.patch: New files.
* Makefile.am (dist_patch_DATA): Add them.

distro: Add GNU bc.

* distro/packages/algebra.scm (bc): New variable.

guix-package: Connect to the daemon only after arguments have been processed.

* guix-package.in (%store): Change to a SRFI-39 parameter.  Update all users.
  (guix-package): Parametrize %STORE to an open connection after
  `parse-options' has been called.

distro: Add Cyrus SASL.

* distro/packages/cyrus-sasl.scm: New file.
* Makefile.am (MODULES): Add it.

Test commit.

* Makefile.am: alphabetical ordering