getopt() returns int

See getopt(3): returns int, so store it in an int variable.
I went through all occurrences, project-wide. All return values of
getopt() should be stored in ints now.

openair2/UTIL/OMG/omg.c

@@ -423,7 +423,7 @@ usage (void) {
 int
 get_options (int argc, char *argv[]) {
   int node_t;
-  char tag;
+  int tag;
 
   while ((tag =
             getopt (argc, argv,