complete rewrite

- add support for authorized_keys options see sshd(8)
- add support for explicit comment after key
- do not allow a key to exist more then once in an authorized_keys file
- remove all conflicting keys

Signed-off-by: Steven Armstrong <steven@icarus.ethz.ch>
new type to manage a single ssh authorized_key entry

Signed-off-by: Steven Armstrong <steven@icarus.ethz.ch>