chg: usr: Harden key management when key files have become unavailabe

Prior to doing key management, BIND 9 will check if the key files on disk match the expected keys. If key files for previously observed keys have become unavailable, this will prevent the internal key manager from running.

Merge branch '4763-do-not-roll-if-key-files-are-missing' into 'main'

See merge request isc-projects/bind9!9337