daemon: Write own daemon routine

RPC will start page-server daemon and needs to get the
controll back to report back to caller, but the glibc's
daemon() does exit() in parent context preventing it.

Thus -- introduce own daemonizing routine.

Strictly speaking, this is not pure daemon() clone, as the
parent process has to exit himself. But this is OK for now.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>