New option "setparms" for 'collectgarbage'

The generational mode also uses the parameters for the incremental
mode in its major collections, so it should be easy to change those
parameters without having to change the GC mode.

testes/gengc.lua

@@ -163,14 +163,15 @@ assert(collectgarbage'isrunning')
 
 
 do  print"testing stop-the-world collection"
-  collectgarbage("incremental", nil, 0)
+  local step = collectgarbage("setparam", "stepsize", 0);
+  collectgarbage("incremental")
 
   -- each step does a complete cycle
   assert(collectgarbage("step"))
   assert(collectgarbage("step"))
 
   -- back to default value
-  collectgarbage("incremental", nil, 200)
+  collectgarbage("setparam", "stepsize", step);
 end
 
 collectgarbage(oldmode)