Switch the memory allocation macros from the Halibut ones

Switch the memory allocation macros from the Halibut ones
(mknew/mknewa/resize) to the PuTTY ones (snew/snewn/sresize). snewn
and mknewa have their arguments opposite ways round; this may make
the change initially painful but in the long term will free me of a
nasty context switch every time I move between codebases. Also
sresize takes an explicit type operand which is used to cast the
return value from realloc, thus enforcing that it must be correct,
and arranging that if anyone tries to compile Halibut with a C++
compiler there should be a lot less pain.

git-svn-id: svn://svn.tartarus.org/sgt/halibut@4276 cda61777-01e9-0310-a592-d414129be87e