socket: add support for systemd sockets

Before opening new IPv4/IPv6 server sockets, chronyd will check for
matching reusable sockets passed from the service manager (for example,
passed via systemd socket activation:
https://www.freedesktop.org/software/systemd/man/latest/sd_listen_fds.html)
and use those instead.

Aside from IPV6_V6ONLY (which cannot be set on already-bound sockets),
the daemon sets the same socket options on reusable sockets as it would
on sockets it opens itself.

Unit tests test the correct parsing of the LISTEN_FDS environment
variable.

Add 011-systemd system test to test socket activation for DGRAM and
STREAM sockets (both IPv4 and IPv6).  The tests use the
systemd-socket-activate test tool, which has some limitations requiring
workarounds discussed in inline comments.

test/system/test.common

@@ -324,7 +324,7 @@ check_chronyd_messages() {
 		([ "$clock_control" -ne 0 ] || grep -q 'Disabled control of system clock' "$logfile") && \
 		([ "$minimal_config" -ne 0 ] || grep -q 'Frequency .* read from' "$logfile") && \
 		grep -q 'chronyd exiting' "$logfile" && \
-		! grep -q 'Could not' "$logfile" && \
+		! (grep -v '^.\{19\}Z D:' "$logfile" | grep -q 'Could not') && \
 		! grep -q 'Disabled command socket' "$logfile" && \
 		test_ok || test_bad
 }