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/nodeUnique.c

@@ -35,6 +35,7 @@
 
 #include "executor/executor.h"
 #include "executor/nodeUnique.h"
+#include "miscadmin.h"
 #include "utils/memutils.h"
 
 
@@ -50,6 +51,8 @@ ExecUnique(UniqueState *node)
 	TupleTableSlot *slot;
 	PlanState  *outerPlan;
 
+	CHECK_FOR_INTERRUPTS();
+
 	/*
 	 * get information from the node
 	 */