ffserver: use correct error for stream not found

Signed-off-by: Reynaldo H. Verdejo Pinochet <reynaldo@osg.samsung.com>
ffserver: drop custom skip_spaces() impl

Signed-off-by: Reynaldo H. Verdejo Pinochet <reynaldo@osg.samsung.com>
doc: mention important API changes in the RELEASE_NOTES

Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>