CLEANUP: http: make it more obvious that msg->som is always null outside of chunks

CLEANUP: http: make it more obvious that msg->som is always null outside of chunks

Since the recent buffer reorg, msg->som is redundant with buf->p but still
appears at a number of places. This tiny patch allows to confirm that som
follows two states :
  - 0 from the moment the message starts to be parsed
  - relative offset to ->p for start of chunk when parsing chunks

During this second state, ->sol is never used, so we should probably merge
the two.