Add support for systemd file descriptor handover in dmeventd.
Systemd preloads file descriptors for us and passes them in for
newly spawned daemon when using on-demand fifo (or socket)
based activation.
This patch adds checks for file descriptors preloaded by
systemd and uses them instead of opening the FIFOs again
to properly support on-demand FIFO-based activation.
(We'll change FIFOs to sockets soon - but still this
part of the code will stay almost the same.)