Check for sd_listen_fds in libsystemd as well as libsystemd-daemon

Upstream systemd has moved the sd_listen_fds function from
libsystemd-daemon into a combined libsystemd. For Debian this change
happens in the 214-1 package. Make the autoconf script look in both
and change the Debian build-deps to look for either suitably
recent libsystemd-dev or fall back to libsystemd-daemon-dev (to aid
backports).