manual: Describe struct link_map, support link maps with dlinfo

manual: Describe struct link_map, support link maps with dlinfo

This does not describe how to use RTLD_DI_ORIGIN and l_name
to reconstruct a full path for the an object. The reason
is that I think we should not recommend further use of
RTLD_DI_ORIGIN due to its buffer overflow potential (bug 24298).
This should be covered by another dlinfo extension.  It would
also obsolete the need for the dladdr approach to obtain
the file name for the main executable.

Obtaining the lowest address from load segments in program
headers is quite clumsy and should be provided directly
via dlinfo.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>