dlwrap: Add new dlwrap_dlopen_libfips function

Previously, two different pieces of fips code (both for dlopen and for
glXGetProcAddress) both needed to dlopen the fips library
itself. However, the two pieces were implemented differently, (one
passed a symbol to dladdr to find a filename to dlopen, the other just
passed NULL to dlsym and hopef for the best).

Make things consistent by having a single, shared implementation in
the new function dlwrap_dlopen_libfips, (and implement it with the
more reliable approach of calling dladdr and then the real dlopen).