Add ability to retrieve default context to util/selinux.rb

Right now we have this great set of utilities for selinux at lib/puppet/util/selinux.rb, and they have some default information. But lib/puppet/type/file/selcontext.rb uses it's own utility to find defaults. We should move that into the util file so we can use it elsewhere as well.

https://github.com/puppetlabs/puppet/blob/51636c065e0625b32af2cd98f2060eb7428704f5/lib/puppet/type/file/selcontext.rb#L40

Namely, we should be able to take advantage of this code in other types that include selinux, like 5klogin.