ionice: add the way to specify the target processes with pgid and uid

ioprio_get and ioprio_set system call accept not only process ID but
also process group ID(pgid) and user ID(uid) to specify the target
process(es).  However, ionice command accepts only process ID.  With
this patch a user can specify the target processes with pgid(-P
option) and uid(-u option).

[kzak@redhat.com: - tiny cleanup in usage()]

Signed-off-by: Masatake YAMATO <yamato@redhat.com>
Signed-off-by: Karel Zak <kzak@redhat.com>