uffdd: Implement --daemon mode

Right now the zdtm.py hacks around core code and waits for
a second for the socket to appear. Let's better make proper
--daemon mode for lazy-pages daemon and pidfile generation.

Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Acked-by: Mike Rapoport <rppt@linux.vnet.ibm.com>