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".