core: add support for naming file descriptors passed using socket activation

This adds support for naming file descriptors passed using socket
activation. The names are passed in a new $LISTEN_FDNAMES= environment
variable, that matches the existign $LISTEN_FDS= one and contains a
colon-separated list of names.

This also adds support for naming fds submitted to the per-service fd
store using FDNAME= in the sd_notify() message.

This also adds a new FileDescriptorName= setting for socket unit files
to set the name for fds created by socket units.

This also adds a new call sd_listen_fds_with_names(), that is similar to
sd_listen_fds(), but also returns the names of the fds.

systemd-activate gained the new --fdname= switch to specify a name for
testing socket activation.

This is based on #1247 by Maciej Wereski.

Fixes #1247.

src/basic/util.c

@@ -6842,3 +6842,28 @@ int version(void) {
              SYSTEMD_FEATURES);
         return 0;
 }
+
+bool fdname_is_valid(const char *s) {
+        const char *p;
+
+        /* Validates a name for $LISTEN_NAMES. We basically allow
+         * everything ASCII that's not a control character. Also, as
+         * special exception the ":" character is not allowed, as we
+         * use that as field separator in $LISTEN_NAMES.
+         *
+         * Note that the empty string is explicitly allowed here.*/
+
+        if (!s)
+                return false;
+
+        for (p = s; *p; p++) {
+                if (*p < ' ')
+                        return false;
+                if (*p >= 127)
+                        return false;
+                if (*p == ':')
+                        return false;
+        }
+
+        return p - s < 256;
+}