rt-sched: Add sched_setattr/sched_getattr API

Until we have a proper libc implementation we maintain a simple
version of it. We this new API we are able to use SCHED_DEADLINE.

This is shamelessly stolen from Dario Faggioli's libdl.

Signed-off-by: Daniel Wagner <daniel.wagner@bmw-carit.de>
Cc: Dario Faggioli <raistlin@linux.it>