[9.20] fix: dev: check plugin config before registering

In `named_config_parsefile()`, when checking the validity of
`named.conf`, the checking of plugin correctness was deliberately
postponed until the plugin is loaded and registered. However,
the checking was never actually done: the `plugin_register()`
implementation was called, but `plugin_check()` was not.

`ns_plugin_register()` (used by `named`) now calls the check function
before the register function, and aborts if either one fails.
`ns_plugin_check()` (used by `named-checkconf`) calls only
the check function.

Backport of MR !11031

Merge branch 'backport-each-check-plugin-named-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11032