core/socket: introduce intermediate SOCKET_START_OPEN state

Prior to this commit, if no Exec*= is defined for socket,
and the unit was in SOCKET_FAILED state, failure of socket_open_fds()
would induce state transition SOCKET_FAILED -> SOCKET_FAILED,
and OnFailure= deps get unexpectedly skipped. Let's introduce
an intermediate state, so that during unit start we enter
UNIT_ACTIVATING at least once.

Fixes #35635

src/basic/unit-def.h

@@ -168,6 +168,7 @@ typedef enum SliceState {
 typedef enum SocketState {
         SOCKET_DEAD,
         SOCKET_START_PRE,
+        SOCKET_START_OPEN,
         SOCKET_START_CHOWN,
         SOCKET_START_POST,
         SOCKET_LISTENING,