Some more small improvements to 'luaO_pushvfstring'

Details:
- counter 'pushed' moved to the struct 'BuffFS'
- new auxiliar function 'getbuff' to build strings directly on
the buffer.

testes/strings.lua

@@ -458,6 +458,9 @@ else
 
   str = string.rep("a", blen - 1) .. "%p" .. string.rep("cd", blen)
   testpfs("P", str, {})
+
+  str = string.rep("%%", 3 * blen) .. "%p" .. string.rep("%%", 2 * blen)
+  testpfs("P", str, {})
 end