cgo: rename reserved field names like type

This commit renames reserved field names like `type` to `_type`, and in
turn renames those fields as well (recursively). This avoids name
clashes when a C struct contains a field named `type`, which is a
reserved keyword in Go.

For some details, see:
https://golang.org/cmd/cgo/#hdr-Go_references_to_C

cgo/testdata/types.go

@@ -15,6 +15,18 @@ typedef struct point3d {
 	int z;
 } point3d_t;
 
+// Structs with reserved field names.
+struct type1 {
+	// All these fields should be renamed.
+	int type;
+	int _type;
+	int __type;
+};
+struct type2 {
+	// This field should not be renamed.
+	int _type;
+};
+
 // Enums. These define constant numbers. All these constants must be given the
 // correct number.
 typedef enum option {
@@ -66,6 +78,10 @@ var (
 	_ C.point3d_t
 	_ C.struct_point3d
 
+	// Structs with reserved field names.
+	_ C.struct_type1
+	_ C.struct_type2
+
 	// Enums (anonymous and named).
 	_ C.option_t
 	_ C.enum_option

cgo/testdata/types.out.go

@@ -72,4 +72,10 @@ type C.struct_point3d struct {
 	y C.int
 	z C.int
 }
+type C.struct_type1 struct {
+	_type   C.int
+	__type  C.int
+	___type C.int
+}
+type C.struct_type2 struct{ _type C.int }
 type C.enum_option C.int