Additional checks for existing names

In ActionElementList, add an additional check for DESCRIPTION_NAME(e)
before comparing with an old footprint.
src/res_parse.y: handle resource globals gracefully.

Likely, res_file is used somewhere to check something. In case a
resource file was loaded earlier, but the current parsing is from
a string, res_file is still != NULL, so this check would mislead.

Solution: Clear res_file and res_filename as soon as the file
          behind it gets closed.

Also replace a number of zeros with the more appropriate NULL.

Affects-bug: lp-1206882
src/action.c: add a number of comments.

... in the hope they're right.
Add intl/plural.c to POTFILES.skip
Make xy file output IPC 7531 compliant.

Improved code style by Bert Timmerman <bert.timmerman@xs4all.nl>.
Regenerate tests after "Make xy file output IPC 7531 compliant.".