chrt: default to SCHED_RR policy

chrt: default to SCHED_RR policy

This fixes a regression introduced in:

commit 7a4ea5664edba98bff28adec3a9c3cfb5763a495
"chrt: add control struct"

Previously (and as documented in the manpage) the default policy
was SCHED_RR. Now it's implicitly SCHED_OTHER (0) as the value
is not initialized explicitly anymore.

Test-command: chrt 90 echo hello

Reported-by: Patrick Pelissier <patrick.pelissier@gmail.com>
Addresses: http://bugs.debian.org/846572
Signed-off-by: Andreas Henriksson <andreas@fatal.se>