Remove the "lone pipe" exception in ShellCommand argument handling

As discussed in
<http://news.gmane.org/find-root.php?group=gmane.comp.python.buildbot.devel&article=10518>.
This makes it possible to pass a pipe as an argument, plus it makes argument
handling consistent between Windows and Unix-likes.
Merge pull request #1234 from SpecLad/remove-pipe-exception

Remove the "lone pipe" exception in ShellCommand argument handling