libpcp: support notify on systemd platforms with no sd_notify
libpcp: support notify on systemd platforms with no sd_notify

Add __pm_sd_notify() for systemd platforms that don't have the
necessary devel package available and so have no sd_notify call.
On such platforms, use an AF_UNIX socket to send the argument
notification message to systemd via the ${NOTIFY_SOCKET} socket.
All the ugly conditional stuff remains in libpcp, so no changes
needed to any of the PCP daemons.

Tested on Fedora by removing the systemd-devel package, rebuilding
and running QA (this platform uses systemd and both pmcd and pmproxy
use systemd units with Type=notify).

adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp