Set usage() to show -d flag
mkesdb supports the -d flag for enabling debug mode, as documented in the manual.

PR: 209865
Submitted by: Maya Rashish <coypu AT sdf DOT org>
Reviewed by: imp
Approved by: bcr (mentor)
Obtained from: NetBSD
MFC after: 7 days
Differential Revision: https://reviews.freebsd.org/D11987