POSIX: px4_getpid() fix

Since the PX4 code uses both px4_task and pthread APIs,
px4_getpid() must be save to call from either context.

On posix, this means we have to always return the pthread ID.

Reverted simulator change of pthread to px4_task

There may have been side effects if this was build for a target that
has process/task scoped file descriptors. It is now safe to call
px4_getpid() from this pthread context with this change for the
posix build for px4_getpid().

Signed-off-by: Mark Charlebois <charlebm@gmail.com>

src/modules/uORB/uORBDevices_posix.hpp

@@ -114,7 +114,7 @@ private:
   uint8_t     *_data;   /**< allocated object buffer */
   hrt_abstime   _last_update; /**< time the object was last updated */
   volatile unsigned   _generation;  /**< object generation count */
-  pid_t     _publisher; /**< if nonzero, current publisher */
+  unsigned long  _publisher; /**< if nonzero, current publisher */
   const int   _priority;  /**< priority of topic */
 
   SubscriberData    *filp_to_sd(device::file_t *filp);