stream: always make stream dumping/capturing append to output file

Partially fixes #1393 (but not really).
command: stip path from playlist OSD display

But only if it's not e.g. a http URL.

Fixes #1388.
manpage: document new --stream-dump behavior

That's probably a good idea.
README: remove travis and coverity badges

Useless and ugly.
screenshots: set AVFrame parameters

Currently, libavcodec is rather lenient, but it might get stricter in
the future.

Fixes #1398.
cocoa: move the gl function loader to gl_cocoa.c
stream_pvr: sort channel list by --tv-channels order

Apparently this is what users would expect.

Going the way of least resistance (in terms of messing with this old,
rarely used code), sorting them by some kind of addition timestamp
(called priority in the patch) is the easiest.

Fixes #1390.
chmap: add a 7.1(rear) layout name

This is common on Apple systems so it's handy to have a label for it.
chmap_sel: add multichannel fallback heuristic

Instead of just failing during channel map selection, try to select a close
layout that makes most sense and upmix/downmix to that instead of failing AO
initialization. The heuristic is rather simple, and uses the following steps:

1) If mono is required always prefer stereo to a multichannel upmix.
2) Search for an upmix that is an exact superset of the required channel map.
3) Search for a downmix that is the exact subset of the required channel map.
4) Search for either an upmix or downmix that is the closest (minimum difference
   of channels) to the required channel map.