Changes so the 'pw' command will allow '$' as the last character in a userid

Changes so the 'pw' command will allow '$' as the last character in a userid
or group name (mainly for the benefit of samba).  This pretty much rewrites
he pw_checkname() routine, but should work exactly the same except for the
above change, and that error messages are somewhat more informative.

PR:		28733 46890
Inspired by:	example patch written by Terry Lambert
Reviewed by:	no objections on freebsd-arch and freebsd-current
MFC plans:	no plans, but will do if people want it in stable.