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.

main.c

@@ -368,9 +368,9 @@ go_daemon(void)
       }
 
       /* Don't keep stdin/out/err from before. But don't close
-         the parent pipe yet. */
+         the parent pipe yet, or reusable file descriptors. */
       for (fd=0; fd<1024; fd++) {
-        if (fd != pipefd[1])
+        if (fd != pipefd[1] && !SCK_IsReusable(fd))
           close(fd);
       }
 
@@ -560,6 +560,9 @@ int main
   if (user_check && getuid() != 0)
     LOG_FATAL("Not superuser");
 
+  /* Initialise reusable file descriptors before fork */
+  SCK_PreInitialise();
+
   /* Turn into a daemon */
   if (!nofork) {
     go_daemon();