Switch from EINHORN_FDS to EINHORN_FD_N/EINHORN_FD_COUNT

The space-separated EINHORN_FDS interface was an attempt to pretend to
be compatible with Upstart. Upstart internally supports passing
multiple FDs, but exposes no interfaces that would allow
it. Meanwhile, having the space-separated list of FDs makes some other
applications more difficult.

@gdb: Mind taking a look?