pid1: rename unit_check_gc to unit_may_gc

"check" is unclear: what is true, what is false? Let's rename to "can_gc" and
revert the return value ("positive" values are easier to grok).

v2:
- rename from unit_can_gc to unit_may_gc

src/core/socket.c

@@ -2819,12 +2819,12 @@ SocketType socket_port_type_from_string(const char *s) {
                 return _SOCKET_TYPE_INVALID;
 }
 
-_pure_ static bool socket_check_gc(Unit *u) {
+_pure_ static bool socket_may_gc(Unit *u) {
         Socket *s = SOCKET(u);
 
         assert(u);
 
-        return s->n_connections > 0;
+        return s->n_connections == 0;
 }
 
 static int socket_accept_do(Socket *s, int fd) {
@@ -3324,7 +3324,7 @@ const UnitVTable socket_vtable = {
         .active_state = socket_active_state,
         .sub_state_to_string = socket_sub_state_to_string,
 
-        .check_gc = socket_check_gc,
+        .may_gc = socket_may_gc,
 
         .sigchld_event = socket_sigchld_event,