Icecast: always send a content-type

use a default (audio/mpeg for historical reason) if none. Required since Icecast 2.4.1
Not using AVOption default because this breaks content-type warnings (needs to
detect if no type was set by the user)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
Icecast: Use 100-continue if possible for proper error handling

Using 100-continue ffmpeg will only send data if the server confirms it,
so if there is an error with auth or mounpoint, this allows that it is
properly reported to the user. Else ffmpeg sends data and just quits at
some point without an error message.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
MAINTAINERS: Add myself as maintainer for Icecast protocol

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
Merge https://github.com/FFmpeg/FFmpeg