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/unit/socket.c

@@ -0,0 +1,61 @@
+/*
+ **********************************************************************
+ * Copyright (C) Luke Valenta  2023
+ * 
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of version 2 of the GNU General Public License as
+ * published by the Free Software Foundation.
+ * 
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ * 
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+ * 
+ **********************************************************************
+ */
+
+#include <socket.c>
+#include "test.h"
+
+static void
+test_preinitialise(void)
+{
+#ifdef LINUX
+  /* Test LISTEN_FDS environment variable parsing */
+
+  /* normal */
+  putenv("LISTEN_FDS=2");
+  SCK_PreInitialise();
+  TEST_CHECK(reusable_fds == 2);
+
+  /* negative */
+  putenv("LISTEN_FDS=-2");
+  SCK_PreInitialise();
+  TEST_CHECK(reusable_fds == 0);
+
+  /* trailing characters */
+  putenv("LISTEN_FDS=2a");
+  SCK_PreInitialise();
+  TEST_CHECK(reusable_fds == 0);
+
+  /* non-integer */
+  putenv("LISTEN_FDS=a2");
+  SCK_PreInitialise();
+  TEST_CHECK(reusable_fds == 0);
+
+  /* not set */
+  unsetenv("LISTEN_FDS");
+  SCK_PreInitialise();
+  TEST_CHECK(reusable_fds == 0);
+#endif
+}
+
+void
+test_unit(void)
+{
+  test_preinitialise();
+}