Introduced wrapper macros snew(), snewn() and sresize() for the

Introduced wrapper macros snew(), snewn() and sresize() for the
malloc functions, which automatically cast to the same type they're
allocating the size of. Should prevent any future errors involving
mallocing the size of the wrong structure type, and will also make
life easier if we ever need to turn the PuTTY core code from real C
into C++-friendly C. I haven't touched the Mac frontend in this
checkin because I couldn't compile or test it.

git-svn-id: http://svn.tartarus.org/sgt/putty@3014 cda61777-01e9-0310-a592-d414129be87e