Allow revoking the access rights

[ePaul]

When I'm done with whatever I had to do, it would be nice if I could tell piu this, which then would revoke the granted access. Something like this:

piu revoke-access ‹host›

This would revoke whatever grant to me was previously created for that host.
There should also be some way to revoke all access grants.

[hjacobs]

@ePaul yes, that's actually a good feature request.

For now you can simply choose a rather small --lifetime (default is only 60 minutes):

# revoke after 20 minutes:
piu req 172.31.1.1 -t 20 "test lifetime"