PHP 7.0 and 7.1: switch to devel/oniguruma6

For mbstring extensions switch from devel/oniguruma5
to devel/oniguruma6 to fix security issues covert in
Oniguruma 6.4.

PHP bundles oniguruma 6.3 but i did some more tests,
to make sure nothing breaks.

Also add $FreeBSD$ header to Makefile.ext

PR: 220598
Submitted by: Trond.Endrestol@ximalas.info
MFH: 2017Q3