Added systemd notify support to EPMD

We already have support for socket activation in EPMD. And this is just another missing piece of systemd support - now a systemd notification subsystem. See this link for further details:

* http://www.freedesktop.org/software/systemd/man/sd_notify.html

Since it's guarded with ifdefs I believe it's harmless for other platforms.

Signed-off-by: Peter Lemenkov <lemenkov@gmail.com>