Revert previous commit: there's an ugly corner-case on windows that fails tests.

Revert previous commit: there's an ugly corner-case on windows that fails tests.

Specifically, test_runner.CreateNode.test_client failed, because the
os.fork-is-present test decided that --multiple should not be allowed on
windows, even though --multiple works just fine for 'tahoe create-client'.
The only restriction on --multiple is for 'tahoe start' and 'tahoe restart'.

This needs a different approach, probably by cleaning up BasedirMixin. We
should only be withholding --multiple on windows for "start" and
"restart". (we should continue withholding --multiple on all platforms for
"run").

This reverts (git) commit f3adb037ae0d22eb06c719c2faef75a834618442:
 "startstop_node.py: fix "tahoe start -m" by forking before non-final targets"