help: Avoid depending on Gnulib mbswidth

help: Avoid depending on Gnulib mbswidth

The Gnulib mbswidth module is total overkill for this application.
Let's not require applications to include it just to support translated
help text.

The kind of problems Gnulib works around should not have any serious
negative effect on the help output, the issues should be limited
to minor alignment problems.  So just add a new configure test to
probe whether mbrtowc and wcwidth are available, use them if they
are, else don't bother.