add openfiles-max and kern.maxfiles settings for OpenBSD

https://trac.torproject.org/projects/tor/wiki/TorRelayGuide/OpenBSD

Cover the following settings:

• openfiles-max
• kern.maxfiles

https://github.com/torbsd/torbsd.github.io/blob/7c20dcfd1176d59d882f1d1a98ca4867976b514c/obsd-relays.md#installation-and-configuration

ref #26619 (moved)