This is a proposed change for addressing GH #2446 "Adding a file with
two ssh-keys can result in an error" (see
https://github.com/dokku/dokku/issues/2446).
The following checks are performed:
- validity of key contents when read from a file
- key contents only contains one single line/key
- authorized_keys file is ultimately valid
The last test is an extension over the previous implementation in that
all lines in the file are checked.
The commit also includes extensions to the relevant test file to
excercise the new checks. It also takes care to save and then restore
the authorized_keys file.
