Fix assorted compiler warnings seen in the buildfarm.

Failure to use DatumGetFoo/FooGetDatum macros correctly, or at all,
causes some warnings about sign conversion.  This is just cosmetic
at the moment but in principle it's a type violation, so clean up
the instances I could find.

autoprewarm.c and sharedfileset.c contained code that unportably
assumed that pid_t is the same size as int.  We've variously dealt
with this by casting pid_t to int or to unsigned long for printing
purposes; I went with the latter.

Fix uninitialized-variable warning in RestoreGUCState.  This is
a live bug in some sense, but of no great significance given that
nobody is very likely to care what "line number" is associated with
a GUC that hasn't got a source file recorded.

src/backend/utils/adt/pgstatfuncs.c

@@ -813,7 +813,7 @@ pg_stat_get_activity(PG_FUNCTION_ARGS)
 					 */
 					nulls[12] = true;
 					nulls[13] = true;
-					values[14] = DatumGetInt32(-1);
+					values[14] = Int32GetDatum(-1);
 				}
 				else
 				{