Create a message descriptor type.

This replaces the sentinel at the end of the pb_field_t
array for each message type.

git-svn-id: https://svn.kapsi.fi/jpa/nanopb-dev@1007 e3a754e5-d11d-0410-8d38-ebb782a927b9

tests/test_decode1.c

@@ -17,7 +17,7 @@ bool print_person(pb_istream_t *stream)
     int i;
     Person person;
     
-    if (!pb_decode(stream, Person_fields, &person))
+    if (!pb_decode(stream, Person_msg, &person))
         return false;
     
     /* Now the decoding is done, rest is just to print stuff out. */