Added control messages to warnings

Added the concept of control messages to the warning system, plus the
implementation of the controls "@on"/"@off" to turn warnings on/off.
Moreover, the warning system in the test library adds some other
controls to ease the test of warnings.

testes/api.lua

@@ -977,6 +977,7 @@ assert(t[7] == nil)
 
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 do   -- testing errors during GC
+  warn("@off")
   collectgarbage("stop")
   local a = {}
   for i=1,20 do
@@ -994,6 +995,7 @@ do   -- testing errors during GC
   collectgarbage()
   assert(A == 10)  -- number of normal collections
   collectgarbage("restart")
+  warn("@on")
 end
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 -- test for userdata vals