ctdb: Improve robust mutex test

ctdb: Improve robust mutex test

This avoids some of the undefined behaviour, like initializing the same mutex
twice which happens when the low and high priority processes start (both
do the initialization and that's dangerous.) Instead now we start an
"init" process to start the shared memory segment, and then everything
else just uses it without truncation or unlinking (same mutex).

Signed-off-by: Carlos O'Donell <codonell@redhat.com>
Reviewed-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>
Autobuild-User(master): Amitay Isaacs <amitay@samba.org>
Autobuild-Date(master): Fri Jun 29 06:47:00 CEST 2018 on sn-devel-144