Fix logging of rekey / trojan invocation delay

Closes #677 (closed)

The rekey / trojan invocation is supposed to happen in the future. Therefore subtract current time to expected time of rekey / invocation, not the reverse.

These delays have been shown incorrectly ever since the SIGUSR1 handler was added in b156b581.

Originally submitted by @wtcline-intc at https://lists.infradead.org/pipermail/openconnect-devel/2024-February/005400.html