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/nodeMergeAppend.c

@@ -40,8 +40,8 @@
 
 #include "executor/execdebug.h"
 #include "executor/nodeMergeAppend.h"
-
 #include "lib/binaryheap.h"
+#include "miscadmin.h"
 
 /*
  * We have one slot for each item in the heap array.  We use SlotNumber
@@ -175,6 +175,8 @@ ExecMergeAppend(MergeAppendState *node)
 	TupleTableSlot *result;
 	SlotNumber	i;
 
+	CHECK_FOR_INTERRUPTS();
+
 	if (!node->ms_initialized)
 	{
 		/*