pgsql: Force plperl and plperlu to run in separate interpreters.

Log Message:
-----------
Force plperl and plperlu to run in separate interpreters. Create an error
on an attempt to create the second interpreter if this is not supported by
the perl installation. Per recent -hackers discussion.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        plperl.sgml (r2.58 -> r2.59)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/plperl.sgml.diff?r1=2.58&r2=2.59)
        release.sgml (r1.482 -> r1.483)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/release.sgml.diff?r1=1.482&r2=1.483)
    pgsql/src/pl/plperl:
        plperl.c (r1.121 -> r1.122)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plperl/plperl.c.diff?r1=1.121&r2=1.122)