maint: add more control flags to mbsalign

maint: add more control flags to mbsalign

* gl/lib/mbsalign.h: Add MBA_UNIBYTE_ONLY (to allow
faster processing).  Also add MBA_NO_LEFT_PAD, MBA_NO_RIGHT_PAD
to give greater control of padding, useful with the first
or last fields on a line.
* gl/lib/mbsalign.c (mbsalign): Implement the new flags.
* gl/tests/test-mbsalign.c (main): Test combinations
of the new flags.