Updates for show single frame.

- allow h264 still images to be shown (based on showiframe patch from
  Erik slagter)
- vusolo2 (and probably dus2 too) did not properly show single
  iframes in enigma2 with drivers from before 2013. This was very
  noticable when in radio mode.
fix typo in decoder.cpp

Thanks IMS
decoder: use writeAll wrapper to write iframe data

Large iframes are not guaranteed to be written with
a single write call.
Deal with incomplete writes.
Prevent user from using fast forward/slow motion in radio recordings

VIDEO_FAST_FORWARD / VIDEO_SLOWMOTION are video device ioctls,
there is no equivalent for the audio device.

Signed-off-by: pieterg <pieterg@users.sourceforge.net>
decoder: fix decoder index handling

redeem ancient FIXME request...

No longer use a 'primary' boolean, just use
the decoder index instead.
introduce common syscall wrappers, and use them

We had an 'eSocketBase' class which contained several
useful wrappers, make them global so everyone can use them.
Typo