ldso: setup search path even when there are no "/"

ldso: setup search path even when there are no "/"

If people use an interp path such as "ld.so", then there is no "/" found,
and we end up dereferencing a NULL pointer.  Simplify the logic by having
a relative interp path like that be the same as "" (which the code later
on interprets as $PWD).

While we're here, document some of the nuances of this code.

Reported-by: Ignacy Gawędzki <uclibc@qult.net>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>