avutil/channel_layout: add av_get_extended_channel_layout

avutil/channel_layout: add av_get_extended_channel_layout

Return a channel layout and the number of channels based on the specified name.

This function is similar to av_get_channel_layout(), but can also parse unknown
channel layout specifications.

Unknown channel layout specifications are a decimal number and a capital 'C'
suffix, in order to not break compatibility with the lowercase 'c' suffix,
which is used for a guessed channel layout with the specified number of
channels.

Signed-off-by: Marton Balint <cus@passwd.hu>