pgsql: Insert conditional SPI_push/SPI_pop calls into InputFunctionCall,

Log Message:
-----------
Insert conditional SPI_push/SPI_pop calls into InputFunctionCall,
OutputFunctionCall, and friends. This allows SPI-using functions to invoke
datatype I/O without concern for the possibility that a SPI-using function
will be called (which could be either the I/O function itself, or a function
used in a domain check constraint). It's a tad ugly, but not nearly as ugly
as what'd be needed to make this work via retail insertion of push/pop
operations in all the PLs.

This reverts my patch of 2007-01-30 that inserted some retail SPI_push/pop
calls into plpgsql; that approach only fixed plpgsql, and not any other PLs.
But the other PLs have the issue too, as illustrated by a recent gripe from
Christian Schrder.

Back-patch to 8.2, which is as far back as this solution will work. It's
also as far back as we need to worry about the domain-constraint case, since
earlier versions did not attempt to check domain constraints within datatype
input. I'm not aware of any old I/O functions that use SPI themselves, so
this should be sufficient for a back-patch.

Modified Files:
--------------
pgsql/src/backend/executor:
spi.c (r1.205 -> r1.206)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/spi.c?r1=1.205&r2=1.206)
pgsql/src/backend/utils/fmgr:
fmgr.c (r1.124 -> r1.125)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/fmgr/fmgr.c?r1=1.124&r2=1.125)
pgsql/src/include/executor:
spi.h (r1.69 -> r1.70)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/executor/spi.h?r1=1.69&r2=1.70)
pgsql/src/pl/plpgsql/src:
pl_exec.c (r1.227 -> r1.228)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_exec.c?r1=1.227&r2=1.228)