When executing rtld directly, allow the value of argv to specify a
file descriptor of the binary to execute. If this fails, fall back to
the previous behaviour of treating argv as a path.
Given the newfound ability of rtld to be directly executed, this approach can be
used as an alternative to creating an ffexecve(2) system call.
This diff has two commits in it: one to rename parse_libdir->parse_integer
and the other to actually do the exec-an-fd work.