Maybe connect() will succeed because it doesn't actually send out any
packets, but sendmsg() fails because the packet couldn't go out for
some reason (e.g., arp failure) or v6 route was lost in the time
between connect() and sendmsg().

heimdal#1304

Noticed as part of:
- heimdal#1286
- heimdal#1304

The duration between consecutive queries to gss_cred_inquire may not
be exactly zero, so the remaining lifetime in seconds may have
decremented by the time we do the second check.

Rather than require instantaneous execution of the test logic, which
was already noted in a comment to be racy, just require that the
change in lifetimes be no more than the change in time elapsed.