* Refactoring: split readkeys() into readfile() and parsekeys()
* Refactoring: split writekeys() into writefile() and serialize()
* authorized_key: support --diff
* Refactoring: remove no-longer used readkeys()/writekeys()
* Integration test for authorized_key in check mode
This change corrects problems reported by the `yamllint` linter.
Since key duplication problems were removed in 4d48711, this commit
mainly fixes trailing spaces and extra empty lines at beginning/end of
files.
