fix: usr: Fix rndc-confgen aborting on HMAC-SHA-384/512 keys above 512 bits

`rndc-confgen -A hmac-sha384` and `-A hmac-sha512` documented a `-b`
range of 1..1024, but any value above 512 aborted on hardened builds
instead of producing a key. The full advertised range now works.

Closes #5903

Merge branch '5903-hmac-generate-stack-overflow' into 'main'

See merge request isc-projects/bind9!11903