Always return pathname in dl_iterate_phdr's dlpi_name, as Linux does

Linux LD_ITERATE_PHDR(3):
    The dlpi_name field is a null-terminated string giving the
    pathname from which the shared object was loaded.

That functionality is much more useful than returning just the short
name.

Update dl_iterate_phdr(3) to follow r272842

MFC of r272842 and r272848