hls: Report the current media sequence

Useful for debugging mostly.
segment: Report the current media sequence

Useful for debugging mostly.
hls: Print start_number as first sequence value

The option now behaves as expected.
hls: Factor the sequence computation
hls: Sync the file number with the start sequence
librtmp: Map native options to librtmp ones when possible

Makes the two protocols nearly seamless.

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>