Add sysctl to make amd64 minidump retry count tunable at runtime.

PR: 213462
Submitted by: RaviPrakash Darbha <rdarbha@juniper.net>
Reviewed by: cemi, markj
Approved by: sjg (mentor)
Obtained from: Juniper Networks
Differential Revision: https://reviews.freebsd.org/D8254