Rename some signal and interrupt handling functions for consistency

The usual pattern for handling a signal is that the signal handler sets 
a global variable and calls SetLatch(MyLatch) to wake up the process, 
and later CHECK_FOR_INTERRUPTS() or other code that is part of a wait 
loop calls another function to deal with it. The naming of the functions 
involved was a bit inconsistent, however. CHECK_FOR_INTERRUPTS() calls 
ProcessInterrupts() to do the heavy-lifting, and the subroutines of 
ProcessInterrupts() were called Process*(), except for 
HandleParallelMessages() and HandleParallelApplyMessages().

Some aux processes don't call CHECK_FOR_INTERRUPTS() and 
ProcessInterrupts() in the main loop, but they have analogous functions 
like HandleMainLoopInterrupts(), HandleStartupProcInterrupts(), etc.

To make things less confusing, the attached patch renames all the 
functions that are part of the overall signal/interrupt handling system 
but are *not* executed in a signal handler to e.g. ProcessSomething(), 
rather than HandleSomething().

Any objections?

P.S. I bumped into this during the larger interrupt revamping work, see 
"Interrupts vs signals" thread. With that, most of the remaining 
functions that are now called Handle*() will go away, and the Process*() 
functions will mostly stay. But IMHO this makes sense independently of 
that work.

-- 
Heikki Linnakangas
Neon (https://neon.tech)