rtld(1): Add shared object search order to the man page

rtld(1): Add shared object search order to the man page

The shared library search order on the dynamic linker was updated a few
weeks ago, but the man page was not updated to reflect its new behavior.
DragonFly's search order is the same as Linux, but differs from FreeBSD
and other BSDs due to the recently added support for DT_RUNPATH and
-z nodefaultlib.

To clarify, FreeBSD and DragonFly *recognized* the DT_RUNPATH dynamic
tag but considered it an alias of DT_RPATH and thus wasn't handling the
tag as intended.

DragonFly is likely is the only BSD that searches shared objects like
Linux, but FreeBSD developers have patches from DragonFly in case they
want to follow suit.