new port: devel/oniguruma6

Oniguruma is a regular expressions library. The characteristics of this
library is that different character encoding for every regular
expression object can be specified.

PR: 212715
Submitted by: yuri@rawbw.com, rob@theseusnetworking.com (earlier version)
Reviewed by: matthew
Approved by: matthew (mentor)
Differential Revision: https://reviews.freebsd.org/D8871