Add support for reloading configuration on SIGHUP

[k0ekk0ek]

Fixes #323.

[orlitzky]

Thanks for this! Since SIGHUP/reload are mentioned in https://github.com/NLnetLabs/nsd/blob/master/doc/README#L607, it might help to compare the two? If I understand correctly, nsd-control reload always reloads everything, but SIGHUP only reloads the zones unless the new option is enabled.

For that matter, it might also make sense to mention the new option in https://github.com/NLnetLabs/nsd/blob/master/nsd.8.in#L209?

[wcawijngaards]

Looks good, the option could also be coded in config_print_zone() in nsd-checkconf.c:285, and config_test_print_server() in nsd-checkconf.c:605 and then see if tpkg/checkconf.tdir needs changes too. Nice to have backwards compatible behaviour with the new option to also read in new zones, drop old zones, and change tsig keys, from the config file.

[k0ekk0ek]

Thanks @wcawijngaards. Forgot about checkconf. I'll add it (and the manpage updates suggested by @orlitzky).

[k0ekk0ek]

@orlitzky

If I understand correctly, nsd-control reload always reloads everything, but SIGHUP only reloads the zones unless the new option is enabled.

No, I hooked up the new option to logic that was already implemented for nsd-control reconfig. Updated the man page.