xapian-bridge C port

endlessm/eos-sdk#2463
`LISTEN_FDS` does not give you the actual FD to listen on, but it will tell you how many FDs systemd is forwarding you starting from number 3 (because 0, 1 and 2 are stdin, stdout and stderr).
We can safely assume that we only want the first one here though, so we don't look at the actual value of the variable. See http://www.freedesktop.org/software/systemd/man/sd_listen_fds.html for more information.
I might dump this rationale in a comment there, as you're not the first person to be confused by this :)