new port: sysutils/siegfried

Siegfried is an advanced file identification tool written in Go.
It is signature-based, implementing the PRONOM file format signatures
and freedesktop.org's MIME-info file format signatures.

PR: 209912
Submitted by: Steffen Fritz <steffen@fritz.wtf>
Reviewed by: matthew
Approved by: matthew (mentor)
Differential Revision: https://reviews.freebsd.org/D9136