interp: fix using Params as an option to New

As correctly pointed out by Marcin Bilski, using the exported function
in that way would lead to a panic. And, even after working around the
panic, any shell flags used would be overwritten by the first implicit
call to Reset.

To fix the crash, move the updateExpandOpts method call to the end of
the "set" builtin. After all, it's only needed there; the Run function
already ends up calling the method later via fillExpandConfig.

To keep the shell options from being deleted in Reset, simply whitelist
them from the list of fields to clear. We do that for Params, so it
follows that the same should happen for the opts field.

Finally, add a couple of tests covering both scenarios to
TestRunnerOpts.

Fixes mvdan#365.

interp/builtin.go

@@ -70,6 +70,7 @@ func (r *Runner) builtinCode(ctx context.Context, pos syntax.Pos, name string, a
 			r.errf("set: %v\n", err)
 			return 2
 		}
+		r.updateExpandOpts()
 	case "shift":
 		n := 1
 		switch len(args) {

interp/interp.go

@@ -262,7 +262,6 @@ func Params(args ...string) func(*Runner) error {
 			args = args[1:]
 		}
 		r.Params = args
-		r.updateExpandOpts()
 		return nil
 	}
 }
@@ -501,6 +500,7 @@ func (r *Runner) Reset() {
 		Exec:        r.Exec,
 		Open:        r.Open,
 		KillTimeout: r.KillTimeout,
+		opts:        r.opts,
 
 		// emptied below, to reuse the space
 		Vars:      r.Vars,

interp/interp_test.go

@@ -2743,6 +2743,16 @@ func TestRunnerOpts(t *testing.T) {
 			"[[ $PWD == foo ]]",
 			"exit status 1",
 		},
+		{
+			opts(Params("foo")),
+			"echo $@",
+			"foo\n",
+		},
+		{
+			opts(Params("-u", "--", "foo")),
+			"echo $@; echo $unset",
+			"foo\nunset: unbound variable\nexit status 1",
+		},
 	}
 	p := syntax.NewParser()
 	for i, c := range cases {