CLEANUP: http: use 'chn' to name channel variables, not 'buf'

CLEANUP: http: use 'chn' to name channel variables, not 'buf'

These "buf" were confusing as they were really refering to channels. At
most places, a buffer was really all what was needed, so a struct buffer
was used instead. It is possible that the performance has slightly increased
by the removal of pointer offset in many pointer operations by directly
using the buffer pointer instead of the channel pointer.