httpd: Start new clients only at keyframes.

When the mux is capable of marking blocks as containing keyframes
(currently only the avformat mux), start new clients only on such
frames. This makes sure they do not get anything they might not
decode at the beginning of the stream.

This patch completes WebM streaming support.

Signed-off-by: Rafaël Carré <funman@videolan.org>
NEWS: mention x262
Contribs: Use x262 from VideoLAN
Mention WebM streaming in NEWS
Win32 installer: do not try to elevate on XP

Should close #9375

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>
Win32: do not map Volume keys by default

On Windows, it seems people expect volume keys to control the master
volume...
I'm not going to fight this fight with our users during support...

Ref #9373
Exclude ALSA from test builds

libasound2 seem to cause spurious test suite failures.
access: avoid genmf and clean up
genmf: remove unused or broken stuff
modules: use different directory prefixes for each plugins subdirectory

This is just a step toward less recursion.
modules: set MODULE_NAME_IS_... where needed only
modules: simplify CPPFLAGS & CFLAGS
modules: remove VLC-specific AM_LIBADD, simplify
access: remove recursion

If you missed the fact that recursive makefiles are harmful, there is
plenty of documentation online (try "Recursive makefiles considered
harmful"), or ask the libav developers for the more concise version.

This change and the previous ones, are particularly boring though. The
main motivation is to eventually address #9397, after the same change is
applied to codec/, demux/ and possibly a few other subdirectories.

Note that some more simplification between Makefile.am files and
configure.ac is definitely possible - if someone is bored.
codec: avoid genmf
codec: remove makefile recursion
demux: avoid genmf

Also rename dirac to diracsys to disambiguate from the encoder.
demux: remove makefile recursion
meta_engine: avoid genmf
meta_engine: remove makefile recursion (refs #9367)