cmd/gc: fix error message for import as 'init'

Fixes #5853.

R=ken2
CC=golang-dev
https://golang.org/cl/11104044

src/cmd/gc/go.y

@@ -197,6 +197,10 @@ import_stmt:
 			importdot(ipkg, pack);
 			break;
 		}
+		if(strcmp(my->name, "init") == 0) {
+			yyerror("cannot import package as init - init must be a func");
+			break;
+		}
 		if(my->name[0] == '_' && my->name[1] == '\0')
 			break;
 		if(my->def) {

src/cmd/gc/y.tab.h

@@ -146,7 +146,7 @@
 
 #if ! defined YYSTYPE && ! defined YYSTYPE_IS_DECLARED
 typedef union YYSTYPE
-#line 28 "go.y"
+#line 30 "go.y"
 {
 	Node*		node;
 	NodeList*		list;

test/fixedbugs/issue4517d.go

+// errorcheck
+
+// Copyright 2012 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+import init "fmt" // ERROR "cannot import package as init - init must be a func"