libvlc: call exit() if command line help was requested

libvlc: call exit() if command line help was requested

A library should not call exit(), but I do not see any easy way around
it: if libvlc_new() returns NULL on error so real errors cannot be
distinguished from --help stuff. Also --daemon already calls exit()
(through daemon()).

This introduces an O(1) memory leak when calling --help. It could be
fixed by merging libvlc_InternalCreate() and libvlc_InternalInit().