code cleanup for mark&sweep GC

doc/advopt.txt

@@ -64,7 +64,8 @@ Advanced options:
   --skipUserCfg             do not read the user's configuration file
   --skipParentCfg           do not read the parent dirs' configuration files
   --skipProjCfg             do not read the project's configuration file
-  --gc:refc|v2|boehm|none   use Nimrod's native GC|V2|Boehm GC|no GC
+  --gc:refc|v2|markAndSweep|boehm|none
+                            select the GC to use; default is 'refc'
   --index:on|off            turn index file generation on|off
   --putenv:key=value        set an environment variable
   --babelPath:PATH          add a path for Babel support