Apparently starting with version 20080615.2 dhcp6c refuses
to run without a dhcp6cctlkey file. Use openssl to generate
one from the rc.d script when it is missing.

PR: 229400
Reviewed by: ler (mentor)
Approved by: maintainer timeout (6 weeks), ler (mentor)
Differential Revision: https://reviews.freebsd.org/D16739