fix: allow to build miniooni for windows (#520)

I need to run test on Windows and I just discovered that:

1. the `errno_unix.go` filename does not mean anything because
`unix` is not a valid platform, so we need a filename for
each platform that we care about;

2. on Windows we need to use WSA prefixed names;

3. `i/e/session_psiphon.go` was not building because of the
migration from `netxlite/iox` to `netxlite`.

This diff attempts to fix all three issues.

The reference issue is https://github.com/ooni/probe/issues/1733,
because I was working on such an issue.

internal/netxlite/errno_linux.go

@@ -0,0 +1,36 @@
+// Code generated by go generate; DO NOT EDIT.
+// Generated: 2021-09-28 18:13:54.230156 +0200 CEST m=+0.321833417
+
+package netxlite
+
+import "golang.org/x/sys/unix"
+
+const (
+	ECONNREFUSED    = unix.ECONNREFUSED
+	ECONNRESET      = unix.ECONNRESET
+	EHOSTUNREACH    = unix.EHOSTUNREACH
+	ETIMEDOUT       = unix.ETIMEDOUT
+	EAFNOSUPPORT    = unix.EAFNOSUPPORT
+	EADDRINUSE      = unix.EADDRINUSE
+	EADDRNOTAVAIL   = unix.EADDRNOTAVAIL
+	EISCONN         = unix.EISCONN
+	EFAULT          = unix.EFAULT
+	EBADF           = unix.EBADF
+	ECONNABORTED    = unix.ECONNABORTED
+	EALREADY        = unix.EALREADY
+	EDESTADDRREQ    = unix.EDESTADDRREQ
+	EINTR           = unix.EINTR
+	EINVAL          = unix.EINVAL
+	EMSGSIZE        = unix.EMSGSIZE
+	ENETDOWN        = unix.ENETDOWN
+	ENETRESET       = unix.ENETRESET
+	ENETUNREACH     = unix.ENETUNREACH
+	ENOBUFS         = unix.ENOBUFS
+	ENOPROTOOPT     = unix.ENOPROTOOPT
+	ENOTSOCK        = unix.ENOTSOCK
+	ENOTCONN        = unix.ENOTCONN
+	EWOULDBLOCK     = unix.EWOULDBLOCK
+	EACCES          = unix.EACCES
+	EPROTONOSUPPORT = unix.EPROTONOSUPPORT
+	EPROTOTYPE      = unix.EPROTOTYPE
+)