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/core/socket.c

@@ -508,6 +508,7 @@ static void socket_dump(Unit *u, FILE *f, const char *prefix) {
                 "%sTCPCongestion: %s\n"
                 "%sRemoveOnStop: %s\n"
                 "%sWritable: %s\n"
+                "%sFDName: %s\n"
                 "%sSELinuxContextFromNet: %s\n",
                 prefix, socket_state_to_string(s->state),
                 prefix, socket_result_to_string(s->result),
@@ -525,6 +526,7 @@ static void socket_dump(Unit *u, FILE *f, const char *prefix) {
                 prefix, strna(s->tcp_congestion),
                 prefix, yes_no(s->remove_on_stop),
                 prefix, yes_no(s->writable),
+                prefix, socket_fdname(s),
                 prefix, yes_no(s->selinux_context_from_net));
 
         if (s->control_pid > 0)
@@ -2760,6 +2762,19 @@ static int socket_get_timeout(Unit *u, uint64_t *timeout) {
         return 1;
 }
 
+char *socket_fdname(Socket *s) {
+        assert(s);
+
+        /* Returns the name to use for $LISTEN_NAMES. If the user
+         * didn't specify anything specifically, use the socket unit's
+         * name as fallback. */
+
+        if (s->fdname)
+                return s->fdname;
+
+        return UNIT(s)->id;
+}
+
 static const char* const socket_exec_command_table[_SOCKET_EXEC_COMMAND_MAX] = {
         [SOCKET_EXEC_START_PRE] = "StartPre",
         [SOCKET_EXEC_START_CHOWN] = "StartChown",