notify: api: allow resetting built-in targets if used by a matcher

notify: api: allow resetting built-in targets if used by a matcher

In the 'delete'-handler targets, we check if a
target is still referenced by a matcher - if it is, we return an
error. For built-in targets, this is actually not necessary, since
'deleting' a built-in only resets it to its default settings - it will
continue to exist after that.
The user could easily trigger this if 'mail-to-root', which is
referenced by 'default-matcher' is modified and then reset to its
defaults: An error is shown, the built-in target is not reset.

This commit disables this check if it is a built-in target.

Renamed the helper 'ensure_unused' to 'ensure_safe_to_delete' in the
process.

Also fixed the tests in api::test - they were never executed due to a
faulty #[cfg] directive.

Signed-off-by: Lukas Wagner <l.wagner@proxmox.com>