Normalize exit codes

[petdance]

The grep docs say:

 The grep utility exits with one of the following values:

 0     One or more lines were selected.
 1     No lines were selected.
 >1    An error occurred.

ack should do the same. I think that right now, only -f respects this, and then not even with an error.

[petdance]

We have a t/exit-code.t but it doesn't check for code 2 = errors, and it's not very exhaustive.