MAJOR: http: change msg->{som,col,sov,eoh} to be relative to buffer origin

MAJOR: http: change msg->{som,col,sov,eoh} to be relative to buffer origin

These offsets were relative to the buffer itself. Now they're relative to
the buffer's origin (buf->p) which normally corresponds to the start of
current message.

This saves a big dependency between the HTTP message struct and the buffers.
It appeared during this change that ->col is not used anymore (it will have
to be removed). Next step is to turn ->eol and ->sol from absolute to relative.