Merge branch 'master' into core-updates

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

* guix/scripts/package.scm: Import (srfi srfi-19).
  (generation-time, matching-generations): New functions.
  (show-help): Add '--list-generations'.
  (%options): Likewise.
  (guix-package)[process-query]: Add support for '--list-generations'.
* guix/ui.scm: Import (srfi srfi-19) and (ice-9 regex).
  (string->generations, string->duration): New functions.
* tests/guix-package.sh: Test '--list-generations'.
* tests/ui.scm: Import (srfi srfi-19).
  Test 'string->generations' and 'string->duration'.
* doc/guix.texi (Invoking guix-package): Document '--list-generations'.

guix-package: Report `--search' matches in recutils format.

* guix/ui.scm (fill-paragraph, string->recutils, package->recutils): New
  procedures.
* guix-package.in (guix-package)[process-query]: Use `package->recutils'
  to display package meta-data.
* tests/guix-package.sh: Adjust test.
* tests/ui.scm: New file.
* Makefile.am (TESTS): Add it.
* doc/guix.texi (Invoking guix-package): Adjust `--search'
  documentation, and give an example.