nss-resolve: expose various source-disablement settings as variables

Fixes https://bugzilla.redhat.com/show_bug.cgi?id=2006761:
> systemd-resolved always (reverse)-resolves the host's IP addresses and FQDN.
> This can be harmful when an application (for instance, a DNS zone manager) is
> installed on the same server instance.  That application would expect
> NXDOMAIN to be returned if the current server's IP does not belong in an
> already managed reverse zone.

This allows clients of nss-resolve to use the same config options that are
available through the dbus api and as command-line options to resolvectl.

The man page text is is mostly copied directly from
c6f20515.