This migrates to a filter_t-like syntax

Open fails if p_sys is NULL.

For lack of vout display currently doing it (obviously ;), it is
untested.

It is needed to avoid problems with threaded event.

(We have a warning for unknown event types anyway)

There is no functionnal changes but it will be needed to reduce locking time
when vout display are allowed to emit events from their own thread.

This slows down the video output thread too much, and does not belong in
the video output plugin anyway. It should go to the screensaver.
(I have also have a patch to move it to xcb_window if someone wants it).

Still plain git-describe is not fixed/fixable, which is hugely annoying
(I use it a lot).

See http://www.rockbox.org/wiki/FmPresets

If people now have "RDS", "hardware frequency seeking" or "VBI capture" in their v4l2 output, please say so!

 * Don't reset cropping when it doesn't make sense (ie fm tuners)
 * When used as an access with the read method, behave as a stream access and not a block access
 * Automatically probe authorised input methods (== if mmap isn't supported but read is, fallback to read)