Fixes make install after 3453a231.

The problem with url_exist() is that it tries to open a resource in
RDONLY mode. If the file is a FIFO and there is already a reading
client, the open() call will hang.

By using avio_check() with access mode of 0, the second reading
process will check if the file exists without attempting to open it,
thus avoiding the lock.

Fix issue #1663.
Signed-off-by: Anton Khirnov <anton@khirnov.net>

Make AVIO_FLAG_ access constants work as flags, and in particular fix
the behavior of functions (such as avio_check()) which expect them to
be flags rather than modes.

This breaks API.

this should've been written earlier, sorry
Signed-off-by: Anton Khirnov <anton@khirnov.net>

This fixes a lot of warnings of the sort:
libavcodec/pcm.c:105: warning: cast discards qualifiers from pointer target type

Automatic stripping of binaries was removed in commit e0be794a
and the _g binaries disappeared along with it.

Use the correct error codes and format identifier.

They've accumulated enough new APIs and corresponding deprecated cruft.

This breaks API and ABI.

The string "FFmpeg" was replaced by "Libav" in metadata that
got encoded in file headers.