interp: support combining pipefail and errexit

We supported both options separately,
but when used together, a command like

    false | true

would fail to exit the shell properly.

Add tests and fix the bug.

Fixes #870.

interp/interp_test.go

@@ -1921,6 +1921,14 @@ var runTests = []runTest{
 		"set -o pipefail; set -M 2>/dev/null | false",
 		"exit status 1",
 	},
+	{
+		"set -o pipefail; false | :; echo next",
+		"next\n",
+	},
+	{
+		"set -e -o pipefail; false | :; echo next",
+		"exit status 1",
+	},
 	{
 		"set -f; >a.x; echo *.x;",
 		"*.x\n",

interp/runner.go

@@ -459,6 +459,7 @@ func (r *Runner) cmd(ctx context.Context, cm syntax.Command) {
 			wg.Wait()
 			if r.opts[optPipeFail] && r2.exit != 0 && r.exit == 0 {
 				r.exit = r2.exit
+				r.shellExited = r2.shellExited
 			}
 			r.setErr(r2.err)
 		}