pgsql: Prepared queries for PLPerl, plus fixing a small plperl memory

Log Message:
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Prepared queries for PLPerl, plus fixing a small plperl memory leak. Patch
and docs from Dmitry Karasik, slightly editorialised.

Modified Files:
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    pgsql/doc/src/sgml:
        plperl.sgml (r2.50 -> r2.51)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/plperl.sgml.diff?r1=2.50&r2=2.51)
    pgsql/src/pl/plperl:
        SPI.xs (r1.18 -> r1.19)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plperl/SPI.xs.diff?r1=1.18&r2=1.19)
        plperl.c (r1.103 -> r1.104)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plperl/plperl.c.diff?r1=1.103&r2=1.104)
        plperl.h (r1.3 -> r1.4)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plperl/plperl.h.diff?r1=1.3&r2=1.4)
    pgsql/src/pl/plperl/expected:
        plperl.out (r1.6 -> r1.7)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plperl/expected/plperl.out.diff?r1=1.6&r2=1.7)
    pgsql/src/pl/plperl/sql:
        plperl.sql (r1.6 -> r1.7)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plperl/sql/plperl.sql.diff?r1=1.6&r2=1.7)