A few more improvements in 'luaO_pushvfstring'

- 'L' added to the 'BuffFS' structure
- '%c' does not handle control characters (it is not its business.
This now is done by the lexer, who is the one in charge of that
kind of errors.)
- avoid the direct use of 'l_sprintf' in the Lua kernel

testes/strings.lua

@@ -453,8 +453,7 @@ else
 
   str = "abc %c def"
   testpfs("I", str, string.byte("A"))
-  -- non-printable character
-  assert(callpfs("I", str, 255) == "abc <\\255> def")
+  testpfs("I", str, 255)
 
   str = string.rep("a", blen - 1) .. "%p" .. string.rep("cd", blen)
   testpfs("P", str, {})