Idempotency tweak to pnacl-helper: don't rm -fr the pnacl toolchain tree.
Idempotency tweak to pnacl-helper: don't rm -fr the pnacl toolchain tree.

Also, improve some comments/help messages, add a clean command.

Review URL: http://codereview.chromium.org/1976001

git-svn-id: 721b910a23eff8a86f00c8fd261a7587cddf18f8@2251 fcba33aa-ac0c-11dd-b9e7-8d5594d729c2

sbc100/native_client,nacl-webkit/native_client,nacl-webkit/native_client,nacl-webkit/native_client,sbc100/native_client,nacl-webkit/native_client,nacl-webkit/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client