PL/pgSQL: Nested CALL with transactions

So far, a nested CALL or DO in PL/pgSQL would not establish a context
where transaction control statements were allowed.  This fixes that by
handling CALL and DO specially in PL/pgSQL, passing the atomic/nonatomic
execution context through and doing the required management around
transaction boundaries.

Reviewed-by: Tomas Vondra <tomas.vondra@2ndquadrant.com>

src/include/tcop/utility.h

@@ -20,6 +20,7 @@ typedef enum
 {
 	PROCESS_UTILITY_TOPLEVEL,	/* toplevel interactive command */
 	PROCESS_UTILITY_QUERY,		/* a complete query, but not toplevel */
+	PROCESS_UTILITY_QUERY_NONATOMIC, /* a complete query, nonatomic execution context */
 	PROCESS_UTILITY_SUBCOMMAND	/* a portion of a query */
 } ProcessUtilityContext;