build: make sure cwd is in Python's sys.path

Apparently some packaging systems (homebrew does this for example) change
`sys.path` before they run any Python script to ensure that only the correct
Python modules can be loadable.

We need to make sure cwd is in `sys.path` since we need to load
`wscript_build.py` from there.
README: mention build result being located at build/mpv
build: fix install path for manual

Appending man1 to the MANDIR was forgotten, so the manual was installed in the
wrong path.
build: fix libavfilter check to include function prototype

Apparently the check error'ed on some compilers for missing prototype and
simply adding one fixes it.
build: use static instead of prototype in libavfilter fragment
build: fix missing prototypes in lua check fragment
build: fix lua check to actually test for libquvi
build: also run the test binary during the lua checks
build: remove unused mng fragment
audio: move decoder context from sh_audio into new struct

Move all state that basically changes during decoding or is needed in
order to manage decoding itself into a new struct (dec_audio).

sh_audio (defined in stheader.h) is supposed to be the audio stream
header. This should reflect the file headers for the stream. Putting the
decoder context there is strange design, to say the least.
audio: don't write decoded audio format to sh_audio

sh_audio is supposed to contain file headers, not whatever was decoded.
Fix this, and write the decoded format to separate fields in the decoder
context, the dec_audio.decoded field. (Note that this field is really
only needed to communicate the audio format from decoder driver to the
generic code, so no other code accesses it.)
player: remove printing of barely correct slave mode stream info

This was printed before something was decoded, and thus was not really
correct. Also, this code is hilariously broken:

        /* Assume FOURCC if all bytes >= 0x20 (' ') */
        if (sh_audio->format >= 0x20202020)
            mp_msg(MSGT_IDENTIFY, MSGL_INFO,
                   "ID_AUDIO_FORMAT=%.4s\n", (char *)&sh_audio->format);

Time to kill it.

This information can be accessed through properties instead.
audio: remove ad_driver.preinit

This never had any real use. Get rid of dec_audio.initialized too, as
it's redundant.
video: move struct mp_hwdec_info into its own header file

This means most code accessing this struct must now include hwdec.h
instead of dec_video.h. I just put it into dec_video.h at first because
I thought a separate file would be a waste, but it's more proper to do
it this way, as there are too many files which include dec_video.h only
to get the mp_hwdec_info definition.
video: merge vd.c into dec_video.c

I don't feel like the separation ever made sense, and it was hard to
tell which file a function you were looking for was in.
options: provide a way for --vf to print custom help

Useful in rather special situations. See following commits.
video: remove vf_pp auto-insertion

This drops the --pp option, which was probably broken for a while. The
option automatically inserted the "pp" filter. The value passed to it
was ignored (which is probably broken, it always selected maximal
quality).

Inserting this filter can be done simply with --vf=pp, so this is not
needed anymore.
options: implement --pphelp differently

Make it work via --vf=pp:help instead.
options: don't prefix sub-options with "--" in help output

Commit 0cb9227a added this to the option list help output, but it looks
strange with sub-options.
options: print lavfi filter list with --vf=lavfi=help