sort(1): Cleanups and a small memory leak.

Remove useless check for leading blanks in the month name. The
code didn't adjust len after stripping blanks so even if a month
*did* start with a blank we'd end up copying garbage at the end.
Also convert a malloc + memcpy to strdup and fix a memory leak in
the wide char version if mbstowcs() fails.
Originally from Andre Smagin.

Obtained from: OpenBSD (CVS rev. 1.2, 1.3)
MFC after: 1 week