POST: handle http/1 pipelined after body

Re-use wsi->preamble_rx to also hold leftover rx after dealing with POST
body.  Ensure ah->rx is always big enough to cope with what may have
been read into the pt->serv_buf.

Update the check for forced needed to also accept non-NULL wsi->preamble
as well as ah->rxpos != ah->rxlen as indication forced needed.

Disable autoservice on ah reset during transaction completed... it may
close the wsi underneath us when it sees and processes the pending
wsi->preamble_rx recursively otherwise.