HAMMER: Add "hammer abort-cleanup" command

HAMMER: Add "hammer abort-cleanup" command

A new command has been added to HAMMER: abort-cleanup.  As could be
deduced from its name, this command will terminate all active cleanup
operations.  It takes no arguments.

It works by searching the /var/run directory for pidfiles starting with
"hammer.cleanup.".  It gets the pid from the filename rather than by
reading the contents of the pidfile.  Once the pid is obtained, the
SIGTERM signal is set to it.  Any successfully terminated process will
be announced on stdout.

The SIGINT signal could also be used, but this one does not print
"Terminated" on the process's terminal, which I find useful.

Currently if the cleanup is interrupted another way, the pidfile will
not be removed.  These hammer.cleanup pidfiles will accumulate until
the next "hammer abort-cleanup" command is executed.  Stale pidfiles
will be removed if encountered by abort-cleanup command.