mountd: Report the absolute path used to load the junction plug-in

As a debugging feature, report the absolute pathname of the plug-in
library that mountd loads to resolve junctions.

Since mountd passes a relative path to dlopen(3), dlopen(3) must
search for the right library.  Displaying the absolute pathname of
the object that it found verifies that mountd loaded the correct
plug-in.

Note: dlinfo(3) is provided by libdl, but there doesn't seem to be a
man page on Fedora 16 for dlinfo(3).  Instead, see:

  http://www.unix.com/man-page/all/3/dlinfo/

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Steve Dickson <steved@redhat.com>