Add passive config option flag to auth command

[llewelld]

Adds a "passive" config option and flag to the auth command. When
enabled the certificates are checked for validity and authentication
happens only if one of the certificates has expired.

[llewelld]

This is very much a work in progress and I'm liable to force push to this branch.

[llewelld]

Contributes to #154.

[llewelld]

It seems to be working now so I've moved this out of draft. Some notes:

1. The flag I've used is passive but it could also be something like force with the opposite semantics?
2. With the additional code the auth branch was getting a bit unwieldy so I moved it into the auth.rs file. I'm not sure whether this fits the intention with the structure, it could go somewhere else or stay in main.rs; please say if I should change this.
3. The code checks whether certificates exist and whether they've expired. Should it be checking anything else?