Add SPI-level support for executing SQL commands with one-time-use plans,

that is commands that have out-of-line parameters but the plan is prepared
assuming that the parameter values are constants.  This is needed for the
plpgsql EXECUTE USING patch, but will probably have use elsewhere.

This commit includes the SPI functions and documentation, but no callers
nor regression tests.  The upcoming EXECUTE USING patch will provide
regression-test coverage.  I thought committing this separately made
sense since it's logically a distinct feature.