Improve handling of dlopen(RTLD_TRACE) aka ldd, of a library that

Improve handling of dlopen(RTLD_TRACE) aka ldd, of a library that
is already loaded:
 * add a 'trace' argument to _dl_show_objects() and exit the
   walk-the-objects loop if you hit that traced object
 * in dlopen(), pass the trace object to _dl_show_objects()
 * also, invoke _dl_show_objects() + exit if the object was
   already opened
 * pass NULL to _dl_show_objects() for all the other calls
 * oh hey, _dl_tracelib is now superfluous: _dl_show_objects()
   should do the walk-the-objects loop only if trace is not NULL.

Problem noted by gnezdo@
ok millert@