rabbitmq-multi would fail to start rabbitmq-server.bat if the path had
parentheses in the first few characters of a path component (but not
after the few few characters, due to short names being involved; so
cases such as 'Program Files (x86)' do not trigger the issue).

This fix makes rabbitmq-multi run rabbitmq-server.bat indirectly via
cmd.exe, working around the erlang issue from bug20526.  So we no
longer need to involve short names, and by sprinkling double quotes in
the right places we can handle parentheses too.

To test this properly you need a path component with both spaces and
parentheses in the first few characters, e.g. "foo ()".