Add support for systemd file descriptor handover in dmeventd.

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.)