Correct file descriptors on Windows

[btoews]

A user reported that signature verification with Git wasn't working on Windows. Git passes --status-fd=1, which isn't the correct file descriptor for stdout on Windows. This RP works around this by special casing file descriptors 1 and 2 to mean stdout and stderr respectively.

[henning-schild]

Closes: #41

[btoews]

Continuing a conversation with @henning-schild from email: With regards to this PR relying on the magic numbers, I don't think there is a great alternative. On Unix systems, we can use syscall.Sydout and syscall.Stderr, but those won't be 1 and 2 on Windows. The problem is that Git always passes --status-fd=1, even on Windows where file descriptor 1 isn't stdout.

[henning-schild]

If you can not access the values of syscall_unix from windows, you could leave a comment or give them names with something like that:

var (
	UnixStdout = 1
	UnixStderr = 2
)

[btoews]

👍 I like that idea.