Make foot able to receive a socket from its parent

We implement the same interface documented in sd_listen_fds, aka,
receive the number of file descriptors passed to the process from the
environment variables LISTEN_FDS.
We only need one, so in every other cases, we still create the socket
ourselves.

+
+    if (listen_pid == NULL || *listen_pid == '\0'
+        || listen_fds == NULL || *listen_fds == '\0')
+        return -1;

+    errno = 0;
+    long pid = strtol(listen_pid, NULL, 10);
+    if (!(errno == 0) || getpid() != pid) {
+        LOG_ERR("Wrong LISTEN_PID environment variable ('%s'), aborting socket activation", listen_pid);

+    }
+
+    long number_of_fds = strtol(listen_fds, NULL, 10);
+    if (!(errno == 0)) {

+    }
+
+    for (int fd = LISTEN_FDS_START;fd < LISTEN_FDS_START + number_of_fds; fd++)
+    {

+    for (int fd = LISTEN_FDS_START;fd < LISTEN_FDS_START + number_of_fds; fd++)
+    {
+        int flags = fcntl(fd, F_GETFD);
+        errno = 0;

+        errno = 0;
+        if (flags == -1) {
+            LOG_ERRNO("Failed to get file descriptors flag for UNIX socket");
+            return (-1);

+            LOG_ERRNO("Failed to get file descriptors flag for UNIX socket");
+            return (-1);
+        }
+        if (fcntl(fd, F_SETFD, flags | FD_CLOEXEC)) {

+    if (listen_fds() == 1)
+    {
+        fd = LISTEN_FDS_START;
+        LOG_INFO("We've been started by socket activation, using passed socket");

+[Service]
+ExecStart=/usr/bin/foot --server

+    long pid = strtol(listen_pid, NULL, 10);
+    if (!(errno == 0) || getpid() != pid) {
+        LOG_ERR("Wrong LISTEN_PID environment variable ('%s'), aborting socket activation", listen_pid);
+        return -1;

+        return -1;
+    }
+
+    for (int fd = LISTEN_FDS_START;fd < LISTEN_FDS_START + number_of_fds; fd++)

+[Service]
+ExecStart=/usr/bin/foot --server
+Environment=WAYLAND_DISPLAY=%i

+	the provided path will be ignored by *foot(1)*.
+	When using systemd socket-activation, you'll need to start (not enable) the
+	socket *foot-server@$WAYLAND\_DISPLAY.socket*.
+

 
+configuration = configuration_data()
+configuration.set('bindir', join_paths(get_option('prefix'), get_option('bindir')))
+

-        goto err;
+    flags = fcntl(LISTEN_FDS_START, F_GETFL);
+    if (flags < 0) {
+        LOG_ERR("Failed to get file status flags for passed socket");

+    }
+    if ((flags & O_NONBLOCK) != O_NONBLOCK) {
+        LOG_ERR("Passed socket is not in non-blocking mode");
+        return false;

+
+    int socket_option = 0;
+    socklen_t len;
+    for (size_t i = 0; i < sizeof(socket_options) / sizeof(*socket_options) ; i++) {

+        len = sizeof(socket_option);
+        if (getsockopt(LISTEN_FDS_START, SOL_SOCKET, socket_options[i], &socket_option, &len) == -1 ||
+                len != sizeof(socket_option)) {
+            LOG_ERR("Failed to read socket option from passed file descriptor");

+    long pid = strtol(listen_pid, NULL, 10);
+    if (errno != 0 || getpid() != pid) {
+        LOG_ERR("Wrong LISTEN_PID environment variable ('%s'), aborting socket activation", listen_pid);
+        return false;

+    char const * const socket_options_names[] = { "SO_DOMAIN", "SO_ACCEPTCONN", "SO_TYPE" };
+
+    assert(ALEN(socket_options) == ALEN(socket_options_values));
+    assert(ALEN(socket_options) == ALEN(socket_options_names));

 	*--server-socket* option in *footclient*(1) and point it to your
 	custom socket path.
 
+	If PATH is a number, foot will interpret it as a socket provided by a

+	use that socket as it's own.
+
+	Two systemd units (foot-server@.{service,socket}) are provided to use that
+	feature with systemd. They need to be instancied with the value of

 want to launch a new terminal.
 
+Foot support  socket activation, which means `foot --server` will only be
+started the first time you'll run `footclient`. (systemd user units are