bstr.c doesn't really deserve its own directory, and compat had just
a few files, most of which may as well be in osdep. There isn't really
any justification for these extra directories, so get rid of them.
The compat/libav.h was empty - just delete it. We changed our approach
to API compatibility, and will likely not need it anymore.
The mp_select_decoders() function doesn't have a log context (and I
don't want to give it one), so get rid of the mp_msg error message by
enhancing the semantics such that the syntax error is replaced by a new
feature. Now doing "--ad=something" will enable all decoders in the
"something" module, same as "--ad=something:*". Pretty useless, but gets
rid of the annoyance.
