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.

socket.h

@@ -73,6 +73,9 @@ typedef struct {
   int descriptor;
 } SCK_Message;
 
+/* Pre-initialisation function */
+extern void SCK_PreInitialise(void);
+
 /* Initialisation function (the specified IP family is enabled,
    or all if IPADDR_UNSPEC) */
 extern void SCK_Initialise(int family);
@@ -106,6 +109,9 @@ extern int SCK_OpenUnixStreamSocket(const char *remote_addr, const char *local_a
                                     int flags);
 extern int SCK_OpenUnixSocketPair(int flags, int *other_fd);
 
+/* Check if a file descriptor was passed from the service manager */
+extern int SCK_IsReusable(int sock_fd);
+
 /* Set and get a socket option of int size */
 extern int SCK_SetIntOption(int sock_fd, int level, int name, int value);
 extern int SCK_GetIntOption(int sock_fd, int level, int name, int *value);