pg_upgrade and PGPORT

pg_upgrade is a bit schizophrenic concerning the PGPORT environment
variable.  On the one hand, there is this code in option.c that wants to
make use of it:

    old_cluster.port = getenv("PGPORT") ? atoi(getenv("PGPORT")) : DEF_PGPORT;
    new_cluster.port = getenv("PGPORT") ? atoi(getenv("PGPORT")) : DEF_PGPORT;
 
On the other hand, check.c will reject a set PGPORT because it's a libpq
environment variable.  Should we make an exception for PGPORT, like we
did for PGCLIENTENCODING?