pgsql: Make pg_regress accept a command-line option for the temporary

Log Message:
-----------
Make pg_regress accept a command-line option for the temporary installation's
port number, and use a default value for it that is dependent on the
configuration-time DEF_PGPORT.  Should make the world safe for running
parallel 'make check' in different branches.  Back-patch as far as 7.4
so that this actually is useful.

Modified Files:
--------------
    pgsql/src/test/regress:
        GNUmakefile (r1.49 -> r1.50)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/GNUmakefile.diff?r1=1.49&r2=1.50)
        pg_regress.sh (r1.58 -> r1.59)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/pg_regress.sh.diff?r1=1.58&r2=1.59)