Add new SPI_OK_REWRITTEN return code to SPI_execute and friends, for the

Add new SPI_OK_REWRITTEN return code to SPI_execute and friends, for the
case that the command is rewritten into another type of command. The old
behavior to return the command tag of the last executed command was
pretty surprising. In PL/pgSQL, for example, it meant that if a command
was rewritten to a utility statement, FOUND wasn't set at all.