Some non-FHS operating systems like NixOS are heavily reliant on
DT_RUNPATH.

Unlike the old DT_RPATH, DT_RUNPATH is relatively simple to implement:
if an object libdependent depends on another object libdependency,
only the DT_RUNPATH of libdependent is used to search for libdependency
(the DT_RUNPATHs of "larger" libraries or the executable are ignored).

DT_RPATH is not implemented here, because it's more difficult: we would
have to do what ld.so does, which is to build up a stack with the
object that loaded the object that loaded (etc.) the object that loaded
the one we are currently considering, and walk all the way up the stack
looking for RPATHs.

Helps: https://github.com/ValveSoftware/steam-runtime/issues/511


Signed-off-by: Simon McVittie <smcv@collabora.com>