Add the option to preload stdnse.make_buffer with data

[nnposter]

Function stdnse.make_buffer provides a very convenient method for reading from a socket in predictable chunks separated by a given pattern, such as one line at a time. This works well when the socket is connected with <socket>:connect but the benefit tends to get substantially reduced when some of the data is incidentally already received from the socket before stdnse.make_buffer has a chance to be called. One such example is when a socket is connected with comm.tryssl, which returns not just the connected socket but also data read by initial <socket>:receive. A user is then left with extracting the chunks from this initial data and gluing them together with subsequent reads from the buffer created with stdnse.make_buffer. The code would generally look something like this:

local socket, data = comm.tryssl(...)
local sep = "\r?\n"
local pos = 1
while pos <= #data do
  local p, q = data:find(sep, pos)
  if not p then break end
  process_line(data:sub(pos, p - 1))
  pos = q + 1
end
local buf = stdnse.make_buffer(socket, sep)
while true do
  local line = buf()
  if not line then break end
  if data then
    line = data:sub(pos) .. line
    data = nil
  end
  process_line(line)
end

This patch is implementing a third parameter for stdnse.make_buffer, allowing this initial, incidentally read data to be returned to the buffer and making it available for subsequent reads. The code then gets substantially simpler:

local socket, data = comm.tryssl(...)
local buf = stdnse.make_buffer(socket, "\r?\n", data)
while true do
  local line = buf()
  if not line then break end
  process_line(line)
end

This PR will be merged in after February 1 unless concerns are raised.