illumos gate

Chris Siden and Delphix (and others?) came up with a nice little script to check the mail_msg file after a build to make sure various sections of it are empty, as they should be for a clean build. I did a little more work to integrate that under $SRC/tools/scripts, along with a minor change to nightly.sh to make the job of this script a little easier (create the mail_msg file before the POST_HOOK is run).