PR26775: make grooming progress visible & interruptible

PR26775: make grooming progress visible & interruptible

On very large servers, it's desirable to be able to interrupt a rescan
or groom cycle.  SIGUSR[12] now do that.  (Unfortunately, this is not
practically testable in the testsuite, since these cycles are so fast
on that small dataset.)  We also expose more internal progress count
about the grooming pass, so the administrator can assess possible need
to interrupt.

Signed-off-by: Frank Ch. Eigler <fche@redhat.com>