Don't lie to mkshort about utf8. (#468)

Don't lie to mkshort about utf8. (#468)

Pass a flag indicating utf8 on every call to mkshort, instead of
setting it up in the handle.  This is much more likely to be correct
as the string and the flag are passed together.  The old method was
often incorrect as it was based on global_opts.charset, which was
often unrelated to the string that was passed to mkshort.

This also resolved an apparent bug in psitrex.  It was thought
that passing a nullptr to mkshort was a bug, but this results in
the result being generated from the default name.  It is assumed
that this is what was originally intended, and this was never a bug.