pgsql: Various small improvements and cleanups for PL/Perl.

Log Message:
-----------
Various small improvements and cleanups for PL/Perl.

- Allow (ineffective) use of 'require' in plperl
    If the required module is not already loaded then it dies.
    So "use strict;" now works in plperl.

- Pre-load the feature module if perl >= 5.10.
    So "use feature :5.10;" now works in plperl.

- Stored procedure subs are now given names.
    The names are not visible in ordinary use, but they make
    tools like Devel::NYTProf and Devel::Cover much more useful.

- Simplified and generalized the subroutine creation code.
    Now one code path for generating sub source code, not four.
    Can generate multiple 'use' statements with specific imports
    (which handles plperl.use_strict currently and can easily
    be extended to handle a plperl.use_feature=':5.12' in future).

- Disallows use of Safe version 2.20 which is broken for PL/Perl.
    http://rt.perl.org/rt3/Ticket/Display.html?id=72068

- Assorted minor optimizations by pre-growing data structures.

Patch from Tim Bunce, reviewed by Alex Hunsaker.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        plperl.sgml (r2.74 -> r2.75)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/plperl.sgml?r1=2.74&r2=2.75)
    pgsql/src/pl/plperl:
        plc_perlboot.pl (r1.2 -> r1.3)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plperl/plc_perlboot.pl?r1=1.2&r2=1.3)
        plc_safe_bad.pl (r1.2 -> r1.3)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plperl/plc_safe_bad.pl?r1=1.2&r2=1.3)
        plc_safe_ok.pl (r1.2 -> r1.3)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plperl/plc_safe_ok.pl?r1=1.2&r2=1.3)
        plperl.c (r1.160 -> r1.161)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plperl/plperl.c?r1=1.160&r2=1.161)
    pgsql/src/pl/plperl/expected:
        plperl.out (r1.17 -> r1.18)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plperl/expected/plperl.out?r1=1.17&r2=1.18)
        plperl_plperlu.out (r1.1 -> r1.2)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plperl/expected/plperl_plperlu.out?r1=1.1&r2=1.2)
    pgsql/src/pl/plperl/sql:
        plperl.sql (r1.15 -> r1.16)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plperl/sql/plperl.sql?r1=1.15&r2=1.16)
        plperl_plperlu.sql (r1.1 -> r1.2)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plperl/sql/plperl_plperlu.sql?r1=1.1&r2=1.2)