grep: Default to -p instead of -S.

This matches the documented behavior in the manpage as well as the
default behavior on macOS.

PR: 280676
Reported by: Radosław Piliszek <radoslaw.piliszek@gmail.com>
Reviewed by: kevans
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D46256

(cherry picked from commit fc12c191c087b63e1204fee210ba76082ea40b96)