guix: lint: Make exception for package name starting description.

* guix/scripts/lint.scm (check-description-style): Exception for
  upper-case rule if the description starts with the package name.
* tests/lint.scm: Test it.

guix: lint: Allow digits at start of synopsis or description.

* guix/scripts/lint.scm (start-with-capital-letter?): Rename too...
  (properly-starts-sentence?): Rewrite with regex and add digits.
  (check-description-style, check-synopsis-style): Use it.
* tests/lint.scm: Add tests.

guix: lint: Check for empty synopses and descriptions.

* guix/scripts/lint.scm (check-description-style,
  check-synopsis-style): New emptiness checks.
* tests/lint.scm: Test them.

guix: lint: Check for proper end-of-sentence space.

* guix/scripts/lint.scm (start-with-capital-letter?): Handle empty
  strings.
  (check-description-style): New check for end-of-sentence space.
* tests/lint.scm: Test it.

guix lint: make sure synopses do not start with the package name.

* guix/scripts/lint.scm (check-start-with-package-name): New method.
* tests/lint.scm ("synopsis: start with package name"): New test.

guix lint: Make sure a synopsis cannot start with a lower-case article.

* guix/scripts/lint.scm (check-start-article): use "string-ci=?" instead of
  "string=?".
* tests/lint.scm ("synopsis: starts with 'a'",
  "synopsis: starts with 'an'"): New tests.

guix lint: check whether descriptions and synopses start with an upper-case letter.

* guix/scripts/lint.scm (check-description-style,
  check-synopsis-start-upper-case): New methods.
* tests/lint.scm ("description: does not start with an upper-case letter",
  "synopsis: does not start with an upper-case letter"): New tests.

guix lint: Make sure synopses are not too long.

* guix/scripts/lint.scm (check-synopsis-length): New procedure.
* tests/lint.scm ("synopsis: too long"): New test.

Merge branch 'master' into core-updates

guix lint: Remove "guix lint: " prefix from warnings.

This allows editors to parse warnings correctly.

* guix/scripts/lint.scm (emit-warning): Use 'format' instead of
  'warning', to avoid the "guix lint: " prefix in messages.
* tests/lint.scm (call-with-warnings): Indent.

scripts: add guix lint

* guix/scripts/lint.scm: New file. Defines a 'lint' tool for Guix packages.
* tests/lint.scm: New file.
* Makefile.am (MODULES, SCM_TESTS): Add them.
* po/guix/Makevars: Update appropriately.
* po/guix/POTFILES.in: Update appropriately.
* doc/guix.texi: Document "guix lint".