Change signature of "exec" and "fg" on Windows to match that on Unix.

This fixes #1195.

pkg/eval/builtin_fn_cmd_unix.go

@@ -27,7 +27,8 @@ var ErrNotInSameProcessGroup = errors.New("not in the same process group")
 // Replace the Elvish process with an external `$command`, defaulting to
 // `elvish`. This decrements `$E:SHLVL` before starting the new process.
 //
-// This command always raises an exception on Windows.
+// This command always raises an exception on Windows with the message "not
+// supported on Windows".
 
 func execFn(fm *Frame, args ...interface{}) error {
 	var argstrings []string

pkg/eval/builtin_fn_cmd_windows.go

@@ -2,13 +2,12 @@ package eval
 
 import "errors"
 
-var (
-	execFn = notSupportedOnWindows
-	fg     = notSupportedOnWindows
-)
-
 var errNotSupportedOnWindows = errors.New("not supported on Windows")
 
-func notSupportedOnWindows() error {
+func execFn(...interface{}) error {
+	return errNotSupportedOnWindows
+}
+
+func fg(...int) error {
 	return errNotSupportedOnWindows
 }