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.
test for and consume warnings about legacy GSP
correctly set the path module for p4 tests on windows

This fixes #2916.
Merge pull request #1237 from djmitche/bug2916

correctly set the path module for p4 tests on windows
Merge pull request #1235 from djmitche/gsp-warnings

test for and consume warnings about legacy GSP
Merge pull request #1234 from SpecLad/remove-pipe-exception

Remove the "lone pipe" exception in ShellCommand argument handling