ipn: move ParseAutoExitNodeID from ipn/ipnlocal to ipn

So it can be used from the CLI without importing ipnlocal.

While there, also remove isAutoExitNodeID, a wrapper around parseAutoExitNodeID
that's no longer used.

Updates tailscale/corp#29969
Updates #cleanup

Signed-off-by: Nick Khyl <nickk@tailscale.com>

ipn/prefs_test.go

@@ -1129,3 +1129,62 @@ func TestPrefsDowngrade(t *testing.T) {
 		t.Fatal("AllowSingleHosts should be true")
 	}
 }
+
+func TestParseAutoExitNodeString(t *testing.T) {
+	tests := []struct {
+		name       string
+		exitNodeID string
+		wantOk     bool
+		wantExpr   ExitNodeExpression
+	}{
+		{
+			name:       "empty expr",
+			exitNodeID: "",
+			wantOk:     false,
+			wantExpr:   "",
+		},
+		{
+			name:       "no auto prefix",
+			exitNodeID: "foo",
+			wantOk:     false,
+			wantExpr:   "",
+		},
+		{
+			name:       "auto:any",
+			exitNodeID: "auto:any",
+			wantOk:     true,
+			wantExpr:   AnyExitNode,
+		},
+		{
+			name:       "auto:foo",
+			exitNodeID: "auto:foo",
+			wantOk:     true,
+			wantExpr:   "foo",
+		},
+		{
+			name:       "auto prefix but empty suffix",
+			exitNodeID: "auto:",
+			wantOk:     false,
+			wantExpr:   "",
+		},
+		{
+			name:       "auto prefix no colon",
+			exitNodeID: "auto",
+			wantOk:     false,
+			wantExpr:   "",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			gotExpr, gotOk := ParseAutoExitNodeString(tt.exitNodeID)
+			if gotOk != tt.wantOk || gotExpr != tt.wantExpr {
+				if tt.wantOk {
+					t.Fatalf("got %v (%q); want %v (%q)", gotOk, gotExpr, tt.wantOk, tt.wantExpr)
+				} else {
+					t.Fatalf("got %v (%q); want false", gotOk, gotExpr)
+				}
+			}
+		})
+	}
+}