We overallocate at the start by LWS_PRE so we can use
buflist content directly with lws_write, this has us also
overallocate by 1 at the end. There's a corner case with
cgi where it's receiving bulk POST data to pass on to cgi
stdin and wants to make sure there's a NUL just past the
end... this makes sure we have space for it.
With http, the protocol doesn't indicate where the headers end and the
next transaction or body begin. Until now, we handled that for client
header response parsing by reading from the tls buffer bytewise.
This modernizes the code to read in up to 256-byte chunks and parse
the chunks in one hit (the parse API is already set up for doing this
elsewhere).
Now we have a generic input buflist, adapt the parser loop to go through
that and arrange that any leftovers are placed on there.
