interface names can include non-word characters

[JensErat]

Interface names are not restricted to word characters. This discussions
indicates pretty arbitrary characters are allowed, checking for non-space
characters should be sufficient for this script.

https://unix.stackexchange.com/q/451368/40798

Additionally, allows setting a different timeout and improves error handling
("bash strict mode") with required changes (default undeclared variables).

If you prefer split commits/PRs, please drop a note or just pick whatever changes you agree to.

[JensErat]

Maybe some remark; this will error out of the script for the first failed wireguard refresh (ie., DNS name not resolveable). This might be desired or not, and can surely be changed by adding some error handling. For me, the "error out" behavior was totally fine, because this way I realize the refresh and wireguard tunnel is broken by looking at the unit state.

[pvcbe]

Hello Jens,
thank you for your pull request.
The script is meant to enhance endpoint uptime, I think the "error out behavior" can indeed prevent this in some cases. For example with a wrong config on one endpoint can block other correct resolving endpoints.
I prefer to not "error out", a syslog message could be more informative and resiliant.
for the rest im good with the pr