Drop the mapping of POSIX function names to the underscored versions, it's

Drop the mapping of POSIX function names to the underscored versions, it's

* glib.h: (Win32) Drop the mapping of POSIX function names to the
underscored versions, it's unnecessary after all. With MSVC we get
them from oldnames.lib, with gcc-2.95 and mingw32 from
-lmoldname-msvc. Add comment about what headers to include for
prototypes.