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 Schröder.

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.