cmd/gc: add diagnostic for var, type, const named init

Before this CL, defining the variable worked fine, but then when
the implicit package-level init func was created, that caused a
name collision and a confusing error about the redeclaration.

Also add a test for issue 3705 (func init() needs body).

Fixes #4517.

R=ken2
CC=golang-dev
https://golang.org/cl/7008045

src/cmd/gc/dcl.c

@@ -188,6 +188,9 @@ declare(Node *n, int ctxt)
 	if(importpkg == nil && !typecheckok && s->pkg != localpkg)
 		yyerror("cannot declare name %S", s);
 
+	if(ctxt == PEXTERN && strcmp(s->name, "init") == 0)
+		yyerror("cannot declare init - must be func", s);
+
 	gen = 0;
 	if(ctxt == PEXTERN) {
 		externdcl = list(externdcl, n);

test/fixedbugs/issue3705.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
+
+func init() // ERROR "missing function body"

test/fixedbugs/issue4517a.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
+
+var init = 1 // ERROR "cannot declare init - must be func"

test/fixedbugs/issue4517b.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
+
+const init = 1 // ERROR "cannot declare init - must be func"

test/fixedbugs/issue4517c.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
+
+type init byte // ERROR "cannot declare init - must be func"