mixer: fix lowering hw volume while muted

Lowering volume while muted did not work correctly with audio outputs
that support native mute setting separate from volume (ao_alsa and
ao_pulse), because the AO-level volume was not set while muted but was
still being read back. Fix by setting the AO volume in this case.
network: fix crash with -playlist http://...

The function fixup_network_stream_cache() accesses stream->opts, which
is NULL in some cases when loading playlists. stream->opts is a pointer
to the global MPOpts struct. For simplicity, this parameter is left
NULL in some cases. Usually, this doesn't matter, because barely
anything in stream/ uses MPOpts anyway.

Hack-fix this for now by not accessing MPOpts and disabling the stream
cache in this case. Calling open_stream() with options==NULL now
basically means: do not use cache settings.
build: remove doc/locale language auto-detection, simplify

This removes the rather complicated configure and Makefile parts
related to auto-detecting available languages for manpages and locales.
We don't have non-English manpages or any locales, so this is
pointless. It didn't even work: configure --language=all created an
invalid config.mak that would cause "make install" to fail.

Remove installation of locales. There are no translations at all which
could be installed. Should there ever be someone who is interested in
adding translations, this can be added back in a simpler way.

Rename the --enable-translation configure option to --enable-gettext.
This is what this option really does: enable gettext() use. This may
be interesting for people who want to experiment with localizing mpv,
but is entirely useless for normal use.

Remove detection of the binary codecs directory in configure.
TOOLS: remove checktree.sh

Nice idea, but essentially useless. Unmaintained even in mplayer-svn.

Remove broken remains of the checkheaders target too.
Makefile: add mpv.rst dependencies

Using a really dumb rule declaration.
build: build manpage by default if rst2man is detected

Add building the manpage to the all target (which is also the default
target). This fixes the behavior that "make install" tried to build the
manpage if it wasn't built yet.

Add rst2man detection to configure, and disable rst2man usage in the all
and install targets if it hasn't been found. You can still build or
install the man page manually (by using the install-mpv-man target),
but the all and install targets won't attempt to use rst2man.

Additionally, building/installing the manpage by default can be
explicitly inhibited using the --disable-manpage configure option.
It's possible to avoid rst2man by using "make mpv install-no-man" as
well.
Improve compatibility with Libav 0.8.4 and ffmpeg 0.11.2

Libav 0.8.4 is ridiculously old (in relative terms), so I don't know
how many things are broken silently.

Encoding is disabled, because the required API hasn't been added yet.
(On the other hand, the old API can't be used in newer versions.)

This should improve compatibility with ffmpeg 0.11.2 as well, which
didn't define AV_CODEC_ID_SUBRIP yet.
cfg-mplayer: do not include encode options when encoding disabled

Normally, we always want to enable encoding, as it uses stock ffmpeg
APIs and has no other dependencies or disadvantages.

However, supporting older releases of ffmpeg and Libav (which equal to
outdated git snapshots fix security and crash fixes applied) force us
to disable some advanced ffmpeg API usage, which includes encoding.
example.conf: all options should be commented

The file is written with the assumption that it doesn't change any
settings by default. Add a forgotten comment to fix this.
Makefile: do not create config directory with install target

The config directory, controlled by --confdir and which is set to
PREFIX/etc/mpv by default, should not be created by default, as
"make install" doesn't copy any files there. The user can still create
a config file manually if system-wide configuration is desired.
video: add IMGFMT_Y16/PIX_FMT_GRAY16

This pixel format is sometimes used with yuv4mpeg.

vo_direct3d used its own IMGFMT_Y16 internally for some reason.

vo_opengl, vo_opengl_old, and vo_direct3d should be able to display
this pixel format natively.
Makefile: don't strip by default, add install-strip targets

Now "make install" will never strip the binary. "make install-strip"
always will.

The behavior of --enable-debug is unchanged, other than having no
influence anymore on the install targets.
options: make --loop always per-file

In this example, only f1.mkv was looped: mpv f1.mkv f2.mkv --loop=2
This is because the playloop actually changes the global option value,
assuming it would be reset when going to the next file. When mpv was
changed to not reset options between files, this assumption was broken.
mplayer: do not freeze when trying to loop an unseekable file

Using --loop=inf on an unseekable file would put mpv (and all other
mplayers as well) into an endless loop, trying to seek to the start of
the file on each playback loop iteration. When the seek fails, playback
simply remains in the at-end-of-file state, and tries to issue a new
seek command for looping.

Fix by checking if the seek command fails, and abort looping in this
case. For that, queue_seek() is replaced with seek(). Due to the
circumstances, these two calls happen to be equal in this case: the
seek is absolute (i.e. no seek coalescing done), and the execution of
queued seeks is right after the loop code anyway.