guix-build: Allow version-qualified package names.

* guix-build.in (guix-build)[find-package]: New procedure.
  Use it instead of using `find-packages-by-name' directly.
  Suggested by Andreas Enge <andreas@enge.fr>.
* tests/guix-build.sh: Add tests.
* doc/guix.texi (Invoking guix-build): Add `coreutils-8.20' as an
  example.  Fix guile-1.8 example.

packages: Have `package-derivation' return a <derivation> as a second value.

* guix/packages.scm (cache): Change the `drv' argument to `thunk'.
  Memoize all the return values of THUNK.
  (cached-derivation): Remove.
  (cached): New macro.
  (package-derivation): Use `cached' instead of `(or (cached-derivation) …)'.
* doc/guix.texi (Defining Packages): Update accordingly.

doc: Run `useradd -g guix-builder -G guix-builder'.

* doc/guix.texi (Setting Up the Daemon): Add `-G guix-builder' when
  invoking `useradd'.
  Suggested by Aleix Conchillo Flaqué <aconchillo@gmail.com>.

doc: Add a "GNU Distribution" node.

* doc/guix.texi (Introduction): Add cross-reference to "GNU
  Distribution".
  (Features): Mention reproducibility.
  (Invoking guix-package): Add cross-reference to "GNU Distribution".
  (GNU Distribution): New node.

doc: Mark binary deployment as not implemented yet.

* doc/guix.texi (Features): Add a footnote saying that binary deployment
  is missing.

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'.

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.

doc: Start documenting (guix store).

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

guix-package: Create or diagnose missing profile directory.

Reported by Andreas Enge.

* guix-package.in (%profile-directory): Honor $NIX_STATE_DIR.
  (guix-package)[ensure-default-profile]: Use it.
  [process-actions]: Call it when the `profile' option is
  %CURRENT-PROFILE.
* tests/guix-package.sh: Add installation test with $HOME set, using the
  default profile.

doc: Fix typos.

* doc/guix.texi (Setting Up the Daemon): Use "useradd -c" for comments.
  Reported by Andreas Enge.
  (Invoking guix-gc): Fix typo.

doc: Mention store sharing with Nix; update `package' example.

* doc/guix.texi: Set @documentlanguage.
  (Installation): Add reference to `INSTALL' and `README'.
  (Requirements): Mention store sharing with Nix.
  (Defining Packages): Update example to include `use-modules' clauses
  and to use (guix licenses).

Merge branch 'core-updates'

Conflicts:
	guix/build/union.scm

distro: libsigsegv: Use a single output.

* distro/packages/libsigsegv.scm (libsigsegv): Use just a single output,
  otherwise nothing ends up in `out' as a consequence of commit a06a99f.
  Reported by Andreas Enge <andreas@enge.fr>.

doc: Clarify package version specification example.

* doc/guix.texi (Invoking guix-package): Use "guile-1.8.8" as the
  example, not "guile-1.8" since there is no such version.

guix-package: Create ~/.guix-profile when it doesn't exist.

* guix-package.in (guix-package): Create the %USER-ENVIRONMENT-DIRECTORY
  symlink if it doesn't exist yet.
* doc/guix.texi (Invoking guix-package): Document it.

guix-package: Show package outputs in `--list-available'.

* guix-package.in (guix-package)[process-query]: For `list-available',
  show the outputs of each package.
* doc/guix.texi (Invoking guix-package): Update accordingly.

doc: Update copyright years; add visible copyright statement.

* doc/guix.texi (YEARS): New variable.
  Use it for all copyright statements.
  (Top): Add a copyright statement and license header here.

doc: Documentation installation and the daemon.

* doc/guix.texi (Installation): New node.