When we reimplemented LLProcess on APR, necessitating APR's funny callback
mechanism to sense child-process status, every isRunning() or getStatus() call
called the APR poll function that calls ALL registered LLProcess callbacks. In
other words, every time any consumer called any LLProcess::isRunning() method,
all LLProcess callbacks were redundantly fired. Change that so that the single
APR poll function is called once per frame, courtesy of the "mainloop"
LLEventPump. Once per viewer frame should be well within the realtime duration
in which it's reasonable to expect child-process status to change.
In effect, this changes LLProcess's public API to introduce a dependency on
"mainloop" ticks. Add such ticks to llprocess_test.cpp as well.