MINOR: channel/buffer: replace buffer_slow_realign() with channel_slow_realign()...

MINOR: channel/buffer: replace buffer_slow_realign() with channel_slow_realign() and b_slow_realign()

Where relevant, the channel version is used instead. The buffer version
was ported to be more generic and now takes a swap buffer and the output
byte count to know where to set the alignment point. The H2 mux still
uses buffer_slow_realign() with buf->o but it will change later.