fix: usr: Validate key names in rndc-confgen, tsig-keygen, ddns-confgen

The three tools embedded the key-name argument verbatim into the
generated `named.conf` block, so a name containing characters like
`"`, `{`, or `;` produced output that did not match the intended
`key` clause. Key names are now restricted to letters, digits, dots,
hyphens, and underscores.

Closes #5904

Merge branch '5904-confgen-keyname-config-injection' into 'main'

See merge request isc-projects/bind9!11904