Patches to xuserrun to fix Issue #2 and another bug.

@Gen2ly opened #2 because of a bug, and pasted his proposed rewrite (not as a commit).  And for simple no-arg commands, it works.

But when I tried his script, I still had problems because it assumed that every argument was a new command to run (and there wasn't an easy way to pass arguments to a single command), so I fixed that in this pull request.

The assumption is that for every `xuserrun` invocation, exactly one command will be run.