[MODCLUSTER-414] Fix compilation warnings in the native part (mod_advertise+mod_manager)

https://issues.jboss.org/browse/MODCLUSTER-414

* i think we may want to analyze if these could not have been more severe bugs

mod_advertise part:
* line 89: unused variable (ma_listen_socket)
* line 178: string literals need to be compared using strcmp()
* line 657: wrong format for port %l but port is defined as int
* line 734: wrong format with precision loss, type long int but printed as %d

mod_manager part:

* fix several problems when uninitialized value was returned
Hm, this return value seems to be used here:

native/mod_manager/mod_manager.c:397:    return (remove_sessionid(sessionidstatsmem, sessionid));