agnocast_kmod: pid-keyed lookups can resolve to the wrong process after pid reuse #1685

The kmod identifies processes by pid, and state such as a publisher_info can outlive its exited process, so once the pid is reused by a new Agnocast process, pid-based lookups like agnocast_find_process_info() resolve to the wrong process. This affects every such lookup, and with #1677 it also makes set_publisher_shm_info() return -ENOENT on RECEIVE/TAKE, which exits the subscriber process. Related: #1610.