Libunit refactoring: port management.

- Changed the port management callbacks to notifications, which e. g. avoids
the need to call the libunit function
- Added context and library instance reference counts for a safer resource
release
- Added the router main port initialization

src/nxt_process.c

@@ -61,7 +61,7 @@ nxt_bool_t  nxt_proc_conn_matrix[NXT_PROCESS_MAX][NXT_PROCESS_MAX] = {
     { 1, 0, 0, 0, 0 },
     { 1, 0, 0, 1, 0 },
     { 1, 0, 1, 0, 1 },
-    { 1, 0, 0, 0, 0 },
+    { 1, 0, 0, 1, 0 },
 };
 
 nxt_bool_t  nxt_proc_remove_notify_matrix[NXT_PROCESS_MAX][NXT_PROCESS_MAX] = {