core: Support task scheduler policies and priorities

No magic here, just fetch info using getpriority and sched_getxxx calls.
Good news is that the mentioned syscalls take pid as argument and do work
with it, i.e. -- no need in parasite help here.

Restore is splitted into prep -- copy sched bits from image on restorer
args -- and the restore itself. It's done to avoid restoring tasks info
with IDLE priority ;) To make restorer not-fail sched bits are validated
for sanity on prep stage.

Minimal sanity test is also there.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

include/syscall-types.h

@@ -10,6 +10,7 @@
 
 #include <sys/time.h>
 #include <arpa/inet.h>
+#include <sched.h>
 
 #include "types.h"