Move compat/ and bstr/ directory contents somewhere else

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.

old-makefile

@@ -146,7 +146,6 @@ SOURCES = audio/audio.c \
           audio/out/ao_pcm.c \
           audio/out/pull.c \
           audio/out/push.c \
-          bstr/bstr.c \
           common/av_common.c \
           common/av_log.c \
           common/codecs.c \
@@ -175,6 +174,7 @@ SOURCES = audio/audio.c \
           input/input.c \
           input/keycodes.c \
           input/pipe.c \
+          misc/bstr.c \
           misc/charset_conv.c \
           misc/dispatch.c \
           misc/rendezvous.c \
@@ -303,7 +303,6 @@ DIRS =  . \
         audio/decode \
         audio/filter \
         audio/out \
-        bstr \
         common \
         compat \
         input \