Various i18n fixes

[pinotree]

This PR does a number of changes & fixes related to i18n, mostly in the build system:

• drop the old messages.po file
• use ngettext() for the Processor/Processors plural form string
• pass -c to msgfmt to ensure .po files are valid (no problems in the current files)
• pass --previous to msgmerge to help translators
• simplify the makefile rules in po/Makefile:
  • avoid hardcoding language names & filenames
  • do not update the pot file on build by default; this may result in a workflow change for maintainers, and the need to check that the messages are up-to-date from time to time
  • respect $(DESTDIR) when installing (follows the common practice, and the existing bits in the rest of linuxlogo)
  • simplify the xgettext invocation

I did not include the update of linux_logo.pot in this PR, nor the update of the translations. I can do that in a followup PR.

See the log message of each commit for longer explanations.