Non-zero exit code when reloading config file fails after SIGHUP.

When reloading the configuration file via the tinc command, the user will get
an error message if reloading has failed. However, no such warning exists when
sending a HUP signal. Previously, tincd would exit in both cases, but with a
zero exit code. Now it will exit with code 1 when reloading fails after a
SIGHUP, but tincd will keep running if it is signaled via the tinc command.
Instead, the tinc command will exit with a non-zero exit code.