Revert "Switch to PCRE grep."

This reverts commit 90a018a8.

Not a clean revert because things have changed slightly, and I'm sticking
with the egrep/fgrep shell scripts for now.

Bug: http://b/111849261
Bug: https://bugs.exim.org/show_bug.cgi?id=2294
Test: manual
Change-Id: I72ae637c84f0eb1c2b5291db73ebff1628d54110