Process structures refactoring in runtime and libunit.

Generic process-to-process shared memory exchange is no more required.  Here,
it is transformed into a router-to-application pattern.  The outgoing shared
memory segments collection is now the property of the application structure.
The applications connect to the router only, and the process only needs to group
the ports.

src/nxt_process.h

@@ -92,7 +92,6 @@ typedef struct {
     nxt_int_t                use_count;
 
     nxt_port_mmaps_t         incoming;
-    nxt_port_mmaps_t         outgoing;
 
     nxt_thread_mutex_t       cp_mutex;