Some renaming and comments to try to clarify builder and jobserver.

The code is still a bit spaghetti-like, especialy when it comes to
redo-unlocked, but at least the new names are slightly more
comprehensible.

redo/cmd_ifchange.py

@@ -42,7 +42,7 @@ def main():
                     f.add_dep('m', t)
                 f.save()
                 state.commit()
-            rv = builder.main(targets, should_build)
+            rv = builder.run(targets, should_build)
         finally:
             try:
                 state.rollback()