job control: rename signal->group_stop and flags to jobctl and update them

    signal->group_stop currently hosts mostly group stop related flags;
    however, it's gonna be used for wider purposes and the GROUP_STOP_
    flag prefix becomes confusing.  Rename signal->group_stop to
    signal->jobctl and rename all GROUP_STOP_* flags to JOBCTL_*.

    Bit position macros JOBCTL_*_BIT are defined and JOBCTL_* flags are
    defined in terms of them to allow using bitops later.

    While at it, reassign JOBCTL_TRAPPING to bit 22 to better accomodate
    future additions.

    This doesn't cause any functional change.

    -v2: JOBCTL_*_BIT macros added as suggested by Linus.

    Signed-off-by: Tejun Heo <tj@kernel.org>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Oleg Nesterov <oleg@redhat.com>