Generate #defines for initializing message structures.

Usage like:
MyMessage foo = MyMessage_init_default;

MyMessage_init_default will initialize to default values defined in .proto.

MyMessage_init_zero will initialize to null/zero values. Same results as {}
or {0}, but will avoid compiler warnings by initializing everything explicitly.

Update issue 79
Status: FixedInGit

tests/basic_buffer/decode_buffer.c

@@ -16,7 +16,7 @@
 bool print_person(pb_istream_t *stream)
 {
     int i;
-    Person person;
+    Person person = Person_init_zero;
     
     if (!pb_decode(stream, Person_fields, &person))
         return false;