guix build: '-e' can be passed a monadic thunk.

* guix/ui.scm (read/eval): New procedure.
  (read/eval-package-expression): Use it.
* guix/scripts/build.scm (derivations-from-package-expressions): Rename to...
  (derivation-from-expression): ... this.  Accept procedures, under the
  assumption that they are monadic thunk.
  (show-help): Adjust accordingly.
  (guix-build): Ditto.
* tests/guix-build.sh: Add test.
* doc/guix.texi (Invoking guix build): Augment description of '-e'.

doc: Mention Linux containers; emphasize reproducible builds.

* doc/guix.texi (Introduction): Use "containers" instead of "chroots".
  (Invoking guix-daemon): Add @cindex entries.  Mention the Linux
  container features.
  (Features): Add sentence on build reproducibility.

guix build: Add '--log-file'.

* guix/scripts/build.scm (show-help): Add '--log-file'.
  (%options): Likewise.
  (guix-build): Set %FILE-PORT-NAME-CANONICALIZATION.  Honor '--log-file'.
* tests/guix-build.sh: Add '--log-file' tests.
* doc/guix.texi (Invoking guix build): Document '--log-file'.

packages: Add a 'snippet' field to <origin>.

* guix/packages.scm (<origin>): Add 'snippet', 'modules', and
  'imported-modules' fields.
  (patch-and-repack): Make 'inputs' a keyword parameter.  Add 'snippet',
  'modules', and 'imported-modules' parameters.  Accept SOURCE as a raw
  file name.  Insert SNIPPET in BUILDER.  Pass IMPORTED-MODULES to
  'build-expression->derivation'.
  (package-source-derivation): Pass the extra arguments to
  'patch-and-repack'.
* tests/packages.scm ("package-source-derivation, snippet"): New test.
* doc/guix.texi (Defining Packages): Mention the 'patches' and 'snippet'
  fields.
  (Invoking guix build): Tell that --source has patches and snippets
  applied.
  (Software Freedom): Mention packages that contain non-free code.

doc: Document current security issue with substitutes.

Suggested by Mark H. Weaver <mhw@netris.org>.

* doc/guix.texi (Features): Add note about unauthenticated binaries.

doc: Improve documentation of '--no-substitutes'.

Suggested by Mark H. Weaver <mhw@netris.org>.

* doc/guix.texi (Invoking guix package): Spell out the --no-substitutes
  documentation.
  (Invoking guix build): Use the more complete --no-substitutes
  documentation.
  (Invoking guix-daemon): Explain the interaction with what clients ask
  for.

doc: Document the syntax of the '--remove' argument.

* doc/guix.texi (Invoking guix package): Document the syntax of
  '--remove' patterns.

doc: Augment the "Porting" section.

Reported by Andreas Enge <andreas@enge.fr>.

* doc/guix.texi (Porting): Document needed changes to (gnu packages
  bootstrap) and (gnu packages linux).

doc: Improve the copyright notice.

doc: Document "guix pull --url".

* doc/guix.texi (Invoking guix pull): Add --url.

Add (guix monads).

* guix/monads.scm: New file.
* tests/monads.scm: New file.
* Makefile.am (MODULES): Add guix/monads.scm.
  (SCM_TESTS): Add tests/monads.scm.
* doc/guix.texi (The Store Monad): New node.
  (The Store): Reference it.

doc: Fix typos.

* doc/guix.texi: Fix typos.

doc: Link from "guix gc" to '--delete-generations'.

* doc/guix.texi (Invoking guix package): Mention that deleting
  generations prevents roll-back.
  (Invoking guix gc): Link to '--delete-generations'.

guix package: '--delete-generations' deletes generations older than specified.

* guix/scripts/package.scm (matching-generations): Add
  'duration-relation' keyword parameter.
  (guix-package)[process-action](delete-generations): Pass
  #:duration-relation >.
* tests/guix-package.sh: Add test.
* doc/guix.texi (Invoking guix package): Clarify the meaning of
  durations for '--list-durations' and '--delete-durations'.

guix package: Add '--delete-generations'.

* guix/scripts/package.scm (switch-to-previous-generation): New function.
  (roll-back): Use the new function instead of 'switch-link'.
  (show-help): Add '--delete-generations'.
  (%options): Likewise.
  (guix-package)[process-actions]: Add 'current-generation-number',
  'display-and-delete', and 'delete-generation'.  Add support for
  '--delete-generations', and reindent the code.
* tests/guix-package.sh: Test '--delete-generations'.
* doc/guix.texi (Invoking guix-package): Document '--delete-generations'.

Merge branch 'master' into core-updates

guix package: Do not list the zeroth generation.

* guix/scripts/package.scm (guix-package)[process-query]: Change
  'list-generation' to not list the zeroth generation.
* tests/guix-package.sh: Test it.
* doc/guix.texi (Invoking guix package): Document it, and use the
  right term when talking about generations.

Merge branch 'master' into core-updates

doc: Document '--list-generations' among the query options.

* doc/guix.texi (Invoking guix package): Move '--list-generations' below
  "In addition to these actions".

guix package: Show most recently installed packages last.

Suggested by Andreas Enge <andreas@enge.fr>.

* guix/scripts/package.scm (guix-package)[list-generations,
  list-installed]: Reverse the result of 'manifest-packages'.
* doc/guix.texi (Invoking guix package): Document the order of packages
  for '--list-generations' and '--list-installed'.