Greetings!

The attached patch add a `--pipe` option to grep. When used, grep
only exits with with nonzero status on error. In particular, it
doesn't signal "match" / "no match" through the exit code.

Here's an example using Bash:

  # enable automatic error handling
  set -eo pipefail
  # grep for issues in a logfile to produce a report
  cat logfile | grep issue | sort --unique

If grep doesn't find "issue" in its input (which is not an error,
obviously), it exits with status 1. Bash interprets this nonzero exit
code as an error and terminates with an error itself.

In order to fix that bug in the above script, you currently have to
replace `grep ...` with `grep ... || [ $? = 1 ]`, which is not really
readable. As alternative, I've implemented a `--pipe` option, which
only returns nonzero on actual errors, but not when there is no match.
This is a bit of a complementary option to `--quiet`.

Open tasks here:
 * FSF paperwork is not finished, so obviously the patch can't be
   applied yet.
 * Should I add a `-p` to complement the long `--pipe`?
 * Should I call it `--pipe` at all? The other alternative I came up
   with was `--filter`. I don't really like either of them very much.


Cheers!

Uli