For consistency, remove references to vlc from libvlc

For consistency, remove references to vlc from libvlc
Also, make it clearer what is shared and instance-specific

So,
libvlc_t ==> libvlc_global_t
vlc_t ==> libvlc_int_t (internal instance)

p_object->p_libvlc ==> p_object->p_libvlc_globale
p_object->p_vlc ==> p_object->p_libvlc

VLC_OBJECT_VLC ==> VLC_OBJECT_LIBVLC

And by the way, there is some cleanup required :) (ie, some things are created in the global object instead of the instance-specific one)