Rename utf2ucs() to utf8_to_unicode(), and export it so it can be used

Rename utf2ucs() to utf8_to_unicode(), and export it so it can be used
elsewhere.

Similarly rename the version in mbprint.c, not because this affects anything
but just to keep the two copies in exact sync.  There was some discussion of
having only one copy in src/port/ instead, but this function is so small
and unlikely to change that that seems like overkill.

Slightly editorialized version of a patch by Joseph Adams.  (The bug-fix
aspect of his patch was applied separately, and back-patched.)