HUP process signal should reload the process

[mortn]

When sending the hickory-dns proces a HUP signal (on POSIX systems), like kill -s HUP [PID of hickory-dns], it should reload the process.

By reload is meant re-read the configuration files (named.toml) as well as zone files without halting operations/responding to client requests.

[djc]

@divergentdave should this be part of #2725?

[divergentdave]

@divergentdave should this be part of #2725?

We don't have a need for config reloading currently.