(Go version) Multi platform build (#319)

pkg/bbutil/umask_posix.go

@@ -0,0 +1,11 @@
+// +build !windows
+
+package bbutil
+
+import "syscall"
+
+// Umask is a no-op on Windows, and calls syscall.Umask on all other
+// systems. On Windows it returns 0, which is a decoy.
+func Umask(mask int) int {
+	return syscall.Umask(mask)
+}