[rbp/omxplayer] Support ff/rew

Big missing feature added for Pi.
It works by increasing the clock and discarding audio when at 2x and 4x.
For rewind and 8x and higher, it uses a different scheme. It pauses video, and single steps.

For both schemes it uses the normal dvd player mechanism of seeking when the clock has diverged by too much.

Notes:
dvdplayer still decodes (then discards) audio when ff/rew.
I'm skipping the decode step (too much cpu at 4x realtime).

I've made GetTime return m_clock.GetClock() when ff/rew. (m_State.time is too jumpy).
Merge pull request #2955 from popcornmix/ff_rew

[rtbp/omxplayer] Support ff/rew
[osx] - don't steal the hw volume keys. Those are controlled by osx and should always be. This reverts part of 6bb569c as we already did in the frodo branch. And now all nyxboard users say booo and go away ;)
[droid] fix path to xbmc depends in packaging Makefile
[WIN32] fixed: support for long paths broke file access on UNC shares
The correct extended length name is \\?\UNC\server\share and not \\?\\server\share