chrt: don't assume SCHED_BATCH and SCHED_IDLE exist

chrt: don't assume SCHED_BATCH and SCHED_IDLE exist

SCHED_FIFO, SCHED_OTHER, SCHED_RR are part of POSIX 1003.1b Process
Scheduling, so it is correct to assume they always exists.

SCHED_BATCH and SCHED_IDLE are Linux specific, we should not assume
they exists.

Defining SCHED_BATCH and SCHED_IDLE to random values (ie the ones found
on Linux systems) is not an option as they may *collide* with the one of
other systems. For example on GNU/kFreeBSD we have:

   #define SCHED_RR        3

and on Linux we have:

   #define SCHED_BATCH     3

[kzak@redhat.com: - add "Linux specific" notes to chrt.1
                  - add a note about BATCH and PR conflict to
                    this commit message]

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Karel Zak <kzak@redhat.com>