Wormhole doesn't start up in supervisor mode on Windows

[mbyczkowski]

We have received a following bug report:

The fly.io's wormhole agent works in "Sidecar" mode, but "Supervisor" mode fails on Windows because it hard-codes "/bin/sh -c" rather than using "cmd.exe /C" on Windows.

Steps to reproduce:

1. download https://s3.amazonaws.com/flyio-wormhole-builds/0.5.35/pkg/wormhole_windows_amd64.exe
2. download https://github.com/WedgeServer/wedge/releases/download/v0.10.9-w/wedge_windows_amd64.exe (Wedge is a fork of Caddy web server)
3. Run the following command:
   .\wormhole_windows_amd64.exe .\wedge_windows_amd64.exe -port 5000
   [Oct 4 11:26:27] INFO process: Starting program: /bin/sh
   [Oct 4 11:26:27] INFO process: Failed to start program: exec: "/bin/sh": file does not exist
   [Oct 4 11:26:27] FATAL wormhole: Error running program: exec: "/bin/sh": file does not exist

[mbyczkowski]

fixed in #25