avio: Add an option 'rw_timeout'

If set non-zero, this limits duration of the retry_transfer_wrapper()
loop, thus affecting ffurl_read*(), ffurl_write(). As soon as
one single byte is successfully received/transmitted, the timer
restarts.

This has further changes by Michael Niedermayer and Martin Storsjö.

Signed-off-by: Martin Storsjö <martin@martin.st>
