Ensure that el-get-cleanup preserves el-get.

Split the package removal logic into a dangerous internal function
which removes everything, and a user-friendly external
function (el-get-cleanup) which ensures that el-get doesn't get nuked.
Simplify the patch to a single function since there's no known use for
the more general internal version.
Merge pull request #1194 from dpkatz/master

Make el-get-cleanup more conservative about not removing el-get itself.