pgsql: Add SPI_push/SPI_pop calls so that datatype input and output

Log Message:
-----------
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.

Modified Files:
--------------
    pgsql/src/pl/plpgsql/src:
        pl_exec.c (r1.185 -> r1.186)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_exec.c.diff?r1=1.185&r2=1.186)