Process port refactoring.

- Introduced nxt_runtime_process_port_create().
- Moved nxt_process_use() into nxt_process.c from nxt_runtime.c.
- Renamed nxt_runtime_process_remove_pid() as nxt_runtime_process_remove().
- Some public functions transformed to static.

This closes #327 issue on GitHub.

src/nxt_process.c

@@ -783,6 +783,17 @@ nxt_user_cred_set(nxt_task_t *task, nxt_user_cred_t *uc)
 }
 
 
+void
+nxt_process_use(nxt_task_t *task, nxt_process_t *process, int i)
+{
+    process->use_count += i;
+
+    if (process->use_count == 0) {
+        nxt_runtime_process_release(task->thread->runtime, process);
+    }
+}
+
+
 void
 nxt_process_port_add(nxt_task_t *task, nxt_process_t *process, nxt_port_t *port)
 {