(grep-use-null-device): New variable.

(grep-use-null-device): New variable.
(grep-command): Mention `grep-use-null-device'.
(grep-compute-defaults): Compute `grep-use-null-device' if necessary.
Make computation of `grep-command' respect `grep-use-null-device'.
(grep): Respect `grep-use-null-device'.
Call `grep-compute-defaults' even if grep-command is set, if
grep-use-null-device is still tentative.