Add -a option to always renew tickets and tokens

Add -a option to always renew tickets and tokens

Add the -a option to k5start and krenew, which says to always try
to renew our tickets (and tokens, if -t) every time we wake up,
regardless of how much time is left on the tickets.  This is useful if
you want to ensure that a certain amount of lifetime always exists
on the tickets, or if you want to ensure aklog gets run, even if
something else is keeping our tickets fresh.

Based on a patch by Andrew Deason.