Remove a redundant variable in favor a better named one
The site-specific extension should also use the rt4 branch

Should have been part of 784eb98.
Use a descriptive temp directory name for easier manual cleanup
Cleanup the tempdir when we die too

The top-level tempdir won't be cleaned up and warning will be issued if
we're still in it.
Apply RT patches found in the extensions

Passing --forward to patch ensures that already applied patches are
ignored.