Add SPI_push/SPI_pop calls so that datatype input and output functions called

by plpgsql can themselves use SPI --- possibly indirectly, as in the case
of domain_in() invoking plpgsql functions in a domain check constraint.
Per bug #2945 from Sergiy Vyshnevetskiy.

Somewhat arbitrarily, I've chosen to back-patch this as far as 8.0.  Given
the lack of prior complaints, it doesn't seem critical for 7.x.