sysutils/debootstrap: use gnugrep

Debootstrap triggers some weird corner case in our base grep(1),
where it's two orders of magnitude slower than it should be.
Fix it by providing it with GNU grep. This reduces the time
to complete 'debootstrap bionic' from ~4m30s to ~1m30s.

Reviewed By: emaste (earlier version)
Sponsored By: EPSRC
Differential Revision: https://reviews.freebsd.org/D30023