Icecast: Send 100-continue header if possible

This allows for proper error reporting. Only do
this for non-legacy requests as only Icecast >2.4.0
will reply with a proper status.
Libav seems to accept both, 100 and 200 status codes, but
let's stay close to spec.

Signed-off-by: Anton Khirnov <anton@khirnov.net>
Icecast: Send content-type in all cases

This is needed because Icecast since version 2.4.1 doesn't default
to audio/mpeg anymore. AVOption default not used here, since a later
check if -content_type is set is performed and would break.

Signed-off-by: Anton Khirnov <anton@khirnov.net>