dlfcn: Implement the RTLD_DI_PHDR request type for dlinfo

The information is theoretically available via dl_iterate_phdr as
well, but that approach is very slow if there are many shared
objects.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Tested-by: Carlos O'Donell <carlos@rehdat.com>
(cherry picked from commit d056c212)