Move interrupt checking from ExecProcNode() to executor nodes.

In a followup commit ExecProcNode(), and especially the large switch
it contains, will largely be replaced by a function pointer directly
to the correct node. The node functions will then get invoked by a
thin inline function wrapper. To avoid having to include miscadmin.h
in headers - CHECK_FOR_INTERRUPTS() - move the interrupt checks into
the individual executor routines.

While looking through all executor nodes, I noticed a number of
arguably missing interrupt checks, add these too.

Author: Andres Freund, Tom Lane
Reviewed-By: Tom Lane
Discussion:
    https://postgr.es/m/22833.1490390175@sss.pgh.pa.us

src/backend/executor/nodeAppend.c

@@ -59,6 +59,7 @@
 
 #include "executor/execdebug.h"
 #include "executor/nodeAppend.h"
+#include "miscadmin.h"
 
 static bool exec_append_initialize_next(AppendState *appendstate);
 
@@ -204,6 +205,8 @@ ExecAppend(AppendState *node)
 		PlanState  *subnode;
 		TupleTableSlot *result;
 
+		CHECK_FOR_INTERRUPTS();
+
 		/*
 		 * figure out which subplan we are currently processing
 		 */