[DRE-maint] [RFC] emulate sd_listen_fds for systemd support

[Christos Trochalakis]

On Sat, Jun 27, 2015 at 04:01:36AM +0000, Eric Wong wrote:
>Unfortunately, testing this is tricky because FD=3
>(SD_LISTEN_FDS_START) tends to be grabbed by (MRI) Ruby 1.9.3
>and onwards for the internal self-pipe.
>
>So for now, I've manually tested this with a systemd installation
>
>Disclaimer: this is also my first experience using systemd
>

I have also tested it and works as expected. Also, hardcoding
SD_LISTEN_FDS_START seems like the best option. I 'd like to see that
applied to master.