Create, and use for all loads of system DLLs, a wrapper function

Create, and use for all loads of system DLLs, a wrapper function
called load_system32_dll() which constructs a full pathname for the
DLL using GetSystemDirectory.

The only DLL load not covered by this change is the one for
gssapi32.dll, because that one's not in the system32 directory.

git-svn-id: http://svn.tartarus.org/sgt/putty@8993 cda61777-01e9-0310-a592-d414129be87e