xargs: support -E/--eof flag

The gnu version of xargs supports a flag `-e`/`-E`/`--eof` to specify a string that, when reached in the input, will cause xargs to stop processing the input. For example, ``` echo "12 34 56 78 90" | xargs -n1 -E 56 echo 12 34 ``` If there is interest in supporting this feature in the findutils implementation of xargs, I have a patch I'm happy to contribute. ``` echo "12 34 56 78 90" | ./target/debug/xargs -n1 -E 56 echo 12 34 ```