new: usr: New "rndc showconf" command

The new `rndc showconf` command prints the running server configuration. There are three options:
- `rndc showconf -user` displays the user configuration (i.e., the contents of `named.conf`).
- `rndc showconf -builtin` displays the default settings, similar to `named -H`.
- `rndc showconf -effective` displays the effective configuration. This is the merged combination of the `-user` and `-builtin` configurations.

Closes #1075

Merge branch 'colin/effective-config-rndc' into 'main'

See merge request isc-projects/bind9!11123