Merge the generated has_<name> fields into a single one.

git-svn-id: https://svn.kapsi.fi/jpa/nanopb-dev@1008 e3a754e5-d11d-0410-8d38-ebb782a927b9

tests/test_decode1.c

@@ -25,7 +25,7 @@ bool print_person(pb_istream_t *stream)
     printf("name: \"%s\"\n", person.name);
     printf("id: %d\n", person.id);
     
-    if (person.has_email)
+    if (Person_has(person, email))
         printf("email: \"%s\"\n", person.email);
     
     for (i = 0; i < person.phone_count; i++)