chrt: make threads aware

chrt: make threads aware

Currently this program works only with the master thread. Add a '-t'
option to propagate changes to the entire group of threads.

Example:

root@offworld:~/projects/util-linux/schedutils# ls /proc/2111/task/
2111  2112  2119  2121  2138  2139  2159  2160
root@offworld:~/projects/util-linux/schedutils# ./chrt -p 2111
pid 2111's current scheduling policy: SCHED_RR
pid 2111's current scheduling priority: 3
root@offworld:~/projects/util-linux/schedutils# ./chrt -t -p 2 2111
root@offworld:~/projects/util-linux/schedutils# ./chrt -p 2112
pid 2112's current scheduling policy: SCHED_RR
pid 2112's current scheduling priority: 2
root@offworld:~/projects/util-linux/schedutils# ./chrt -p 2111
pid 2111's current scheduling policy: SCHED_RR
pid 2111's current scheduling priority: 2

[kzak@redhat.com: - rename -t/--thread to -a/--all-tasks]

Signed-off-by: Davidlohr Bueso <dave@gnu.org>
Signed-off-by: Karel Zak <kzak@redhat.com>