RJK's OS X portability patch:

 - initialise blank mbstate_t using memset rather than an ad-hoc
   initialiser.
 - expand the OMIT_UTMP ifdefs to enclose a load of entire functions
   that would generate `static function never called' warnings if
   left as empty shells.
 - couple of other fiddly things.

git-svn-id: svn://svn.tartarus.org/sgt/putty@4896 cda61777-01e9-0310-a592-d414129be87e