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.

contrib/jsonb_plpython/jsonb_plpython.c

@@ -325,8 +325,13 @@ PLyNumber_ToJsonbValue(PyObject *obj, JsonbValue *jbvNum)
 
 	PG_TRY();
 	{
-		num = DatumGetNumeric(DirectFunctionCall3(numeric_in,
-												  CStringGetDatum(str), 0, -1));
+		Datum		numd;
+
+		numd = DirectFunctionCall3(numeric_in,
+								   CStringGetDatum(str),
+								   ObjectIdGetDatum(InvalidOid),
+								   Int32GetDatum(-1));
+		num = DatumGetNumeric(numd);
 	}
 	PG_CATCH();
 	{