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.

[originally from svn r8993]