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/all.lua

@@ -209,6 +209,10 @@ if #msgs > 0 then
   warn("#tests not performed:\n  ", m, "\n")
 end
 
+warn("@off")
+warn("******** THIS WARNING SHOULD NOT APPEAR **********")
+warn("******** THIS WARNING ALSO SHOULD NOT APPEAR **********")
+warn("@on")
 print("(there should be two warnings now)")
 warn("#This is ", "an expected", " warning")
 warn("#This is", " another one")