[9.18] fix: usr: DSYNC record incorrectly used two octets for the Scheme Field

When creating the `DSYNC` record from a structure, `uint16_tobuffer` was used instead of `uint8_tobuffer` when adding the scheme, causing a `DSYNC` record that was one octet too long. This has been fixed.

Closes #5711

Backport of MR !11477

Merge branch 'backport-5711-dsync_fromstruct-produces-an-invalid-record-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!11484