Fix logging of rekey / trojan invocation delay

Fix logging of rekey / trojan invocation delay

Closes #677

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

These delays have been shown incorrectly ever since the SIGUSR1 handler was
added in b156b581e894b03e7169827b9e293ca2f13e1366.

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

Signed-off-by: Cline, Wade <wade.cline@intel.com>
Signed-off-by: Dimitri Papadopoulos <3350651-DimitriPapadopoulos@users.noreply.gitlab.com>
Signed-off-by: Daniel Lenski <dlenski@gmail.com>