Variables callback can be (and usually are) invoked from other threads.
They cannot access the object without locking, and must be unregistered
before the object is destroyed (var_DelCallback() -> free(p_sys)).

(Currently, <1ms on Linux, and <0 on others)

Thanks Courmisch for pointing it.

This fixes a few "deadlocks" with input waiting for playlist, while
playlist is waiting for input event.

Also do not needlessly lock after unregistering the callbacks. We have
to assume that when var_DelCallback returns, any pending callback
invocation has be completed. Otherwise, we would any crash when Close()
releases the mutex, destroys it, and the callback then tries to acquire
the mutex.

See page 66 of ETSI EN 300 468

This way the TS demux can create multiple tracks that will take you straight to the subtitles, program listings etc.

So that whatis is happy
Drop some unusefull undefined macros

This prevents the libvlc object from having it's name set to
"memcpymmxext" by module_Need.
This reverts commit 15e7d182.

Casting to any other type would be (is?) unsafe as users can now freely
alias objects.

GCC will print a warning if the result of such function is ignored
This is useful for functions returning an allocated object (which
would leak if ignored), "getter" functions, and functions that are
"likely" to fail.