enhance SPI to support EXECUTE commands

Dear hackers,

I found that such a statement would get 0 in PL/pgSQL.

PREPARE smt_del(int) AS DELETE FROM t1;
EXECUTE 'EXECUTE smt_del(100)';
GET DIAGNOSTICS j = ROW_COUNT;

In fact, this is a problem with SPI, it does not support getting result 
of the EXECUTE command. I made a little enhancement. Support for the 
number of rows processed when executing INSERT/UPDATE/DELETE statements 
dynamically.

Regards,
Quan Zongliang