Remove the _ macro

This was a gettext-style macro to mark strings that should be
translated.
stream: move O_BINARY dummy definition
Reduce recursive config.h inclusions in headers

In my opinion, config.h inclusions should be kept to a minimum. MPlayer
code really liked including config.h everywhere, though, even in often
used header files. Try to reduce this.
m_option: don't include config.h in header

Requires a small workaround.
ad_lavc: work around deprecation warning

request_channels has been deprecated for years (request_channel_layout
is the replacement), but it appears it's still needed despite the
deprecation at least on older libavcodec versions.

So still set request_channels, but to it with the avoption API, which
hides the deprecation warning. This should also prevent mpv getting
trashed when libavcodec happens to bump its major version.
vf_lavfi: don't access AVFilterPad directly

Direct access is deprecated.
ao_wasapi: fix includes

Broken due to recent header renaming. Untested.