[9.18] fix: dev: Fix an issue with selfsigned_dnskey() return value

The selfsigned_dnskey() function currently returns boolean. There
was a recent change to make it return a isc_result_t error code,
which is implicitly converted to bool, which is obviously an error.

If instead of the result code we return true/false, it still doesn't
indicate the error to the caller that has happened before.

Change the function to return isc_result_t, and change the caller
routine to process the new return type.

Closes isc-projects/bind9#5343

Merge branch 'aram/security-selfsigned_dnskey-return-type-fix' into 'v9.18.41-release'

See merge request isc-private/bind9!869