Tighten string-related code requirements in the GPU fakeshader

[y5nw]

First commit: makes the fakeshader GPU command reject string concatenation to avoid issues arising from concatenating long strings (see #85).
Second commit: disallows constructing strings using the single quote (') within the fakeshader command.

Also note that string concatenation can be used to construct certain strings from non-string values (e.g. (1)..(1)).

How to test: use the following Luacontroller code and observe that the GPU rejects the fakeshader code.

if event.type ~= "program" then return end

digiline_send("gpu", {
  {command="createbuffer", buffer=0, xsize=16, ysize=16, fill="000000"},
  {command="fakeshader", buffer=0, code=[[
return ((99)..9):byte(1,3)
]]},
  {command="send", buffer=0, channel="screen"},
})