Clang, like LCC, objects to using '<' and '>' on function pointers.

Clang, like LCC, objects to using '<' and '>' on function pointers.

I'm not entirely sure that using memcmp() is any more defined by the C
standard, but at least Clang doesn't complain about it.  While I'm
here, tree234 doesn't require that comparison functions return
precisely +1 or -1, so we can use the return value of memcmp()
directly.

git-svn-id: svn://svn.tartarus.org/sgt/putty@9636 cda61777-01e9-0310-a592-d414129be87e