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.