Avoid passing function pointers across process boundaries.

We'd already recognized that we can't pass function pointers across process
boundaries for functions in loadable modules, since a shared library could
get loaded at different addresses in different processes.  But actually the
practice doesn't work for functions in the core backend either, if we're
using EXEC_BACKEND.  This is the cause of recent failures on buildfarm
member culicidae.  Switch to passing a string function name in all cases.

Something like this needs to be back-patched into 9.6, but let's see
if the buildfarm likes it first.

Petr Jelinek, with a bunch of basically-cosmetic adjustments by me

Discussion: https://postgr.es/m/548f9c1d-eafa-e3fa-9da8-f0cc2f654e60@2ndquadrant.com

src/backend/access/transam/README.parallel

@@ -198,7 +198,7 @@ pattern looks like this:
 
 	EnterParallelMode();		/* prohibit unsafe state changes */
 
-	pcxt = CreateParallelContext(entrypoint, nworkers);
+	pcxt = CreateParallelContext("library_name", "function_name", nworkers);
 
 	/* Allow space for application-specific data here. */
 	shm_toc_estimate_chunk(&pcxt->estimator, size);