Per discussion on #3359, revert some of the clobber protection

* Revert "Keep from clobbering PAGER and LESS if they're already set"
* Revert "Don't clobber LSCOLORS if its already set"
* Revert "Protect GREP_OPTIONS from getting clobbered"

This reverts commit 1f5cecee4768be192e439a72a873a6cfe8720927.