In TClingCallFunc use new TMutex::Reset/Restore() to clear any and all locks active before entering user code.

This is kosher because the locks were taken to forbid other threads from modifying state while this thread is modifying state.
But this thread will not modify any state while the user code is active; thus while that user code is active,
the mutex can be completely reset, and restored once the user code returns.

See 6568fb2f

Introduce macro R__LOCK_SUSPEND.