Add support for systemd socket activation (where systemd creates the Unix

domain socket for tmux rather than tmux creating it). Build with
--enable-systemd. From Julien Moutinho in GitHub issue 3119.

compat/systemd.c

@@ -0,0 +1,58 @@
+/* $OpenBSD$ */
+
+/*
+ * Copyright (c) 2022 Nicholas Marriott <nicholas.marriott@gmail.com>
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF MIND, USE, DATA OR PROFITS, WHETHER
+ * IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING
+ * OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+#include <sys/types.h>
+#include <sys/un.h>
+
+#include <systemd/sd-daemon.h>
+
+#include "tmux.h"
+
+int
+systemd_create_socket(int flags, char **cause)
+{
+	int			fds;
+	int			fd;
+	struct sockaddr_un	sa;
+	int			addrlen = sizeof sa;
+
+	fds = sd_listen_fds(0);
+	if (fds > 1) { /* too many file descriptors */
+		errno = E2BIG;
+		goto fail;
+	}
+
+	if (fds == 1) { /* socket-activated */
+		fd = SD_LISTEN_FDS_START;
+		if (!sd_is_socket_unix(fd, SOCK_STREAM, 1, NULL, 0)) {
+			errno = EPFNOSUPPORT;
+			goto fail;
+		}
+		if (getsockname(fd, (struct sockaddr *)&sa, &addrlen) == -1)
+			goto fail;
+		socket_path = xstrdup(sa.sun_path);
+		return (fd);
+	}
+
+	return (server_create_socket(flags, cause));
+
+fail:
+	if (cause != NULL)
+		xasprintf(cause, "systemd socket error (%s)", strerror(errno));
+	return (-1);
+}