grep: add -R as well as -r.

On BSD these are actually the same, and there's a -S that you need in
addition. So strictly this is a behavior change for Android (which is
going from BSD grep to toybox grep), but it's a behavior preserving
change for the AOSP build (which is going from GNU grep to toybox grep),
and the latter actually has a checked-in use of -R where the former
2 files changed