domain constraints and UNKNOWN params

David Wheeler reported the following bug: when a protocol-level prepared
statement with a parameter of UNKNOWN type is used, any domain
constraints that are associated with the inferred type of the parameter
are not checked when the statement is executed. Attached is a script
David sent me to reproduce the problem: when pg_server_prepare is
enabled (the default), DBD::Pg uses protocol-level prepared statements,
and the script is able to insert a tuple that violates the domain's
check constraint. Disabling pg_server_prepare results in a constraint
failure, as it should.

I've also attached a patch that should fix the issue -- coerce_type()
neglected to apply coerce_to_domain() to the type inferred for an
UNKNOWN Param. Barring any objections, I intend to apply the patch to
HEAD and release branches as far back as the problem exists (likely 8.0
and 8.1, and possibly 7.4 -- I haven't checked yet).

It would be nice to add regression tests for this issue, but AFAICS
there's no way to specify parameters of UNKNOWN type to a prepared
statement created via SQL -- which might be worth implementing anyway...

-Neil