pg_upgrade libraries check

pg_upgrade is a little over-keen about checking for shared libraries 
that back functions. In particular, it checks for libraries that support 
functions created in pg_catalog, even if pg_dump doesn't export the 
function.

The attached patch mimics the filter that pg_dump uses for functions so 
that only the relevant libraries are checked.

This would remove the need for a particularly ugly hack in making the 
9.1 backport of JSON binary upgradeable.

cheers

andrew