cmp{p,s}{s,d} instructions do take an imm8 operand.
Signed-off-by: Diego Biurrun <diego@biurrun.de>

af_channelmap: fix uninitialized variable use introduced in ba8efac9

Useful for examining options, the same as the corresponding functions
for the other libs.

Change the mappings separator from comma to '|' to avoid excessive
escaping, since comma is already used for separating filters in the
filtergraph description.