pgsql: Rationalise perl header inclusions via a common include file,

Log Message:
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Rationalise perl header inclusions via a common include file, which also
declares routines in plperl.c and spi_internal.c used in other files.

Along the way, also stop perl from hijacking stdio and other stuff on Windows.

Modified Files:
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    pgsql/src/pl/plperl:
        SPI.xs (r1.17 -> r1.18)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plperl/SPI.xs.diff?r1=1.17&r2=1.18)
        plperl.c (r1.98 -> r1.99)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plperl/plperl.c.diff?r1=1.98&r2=1.99)
        spi_internal.c (r1.7 -> r1.8)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plperl/spi_internal.c.diff?r1=1.7&r2=1.8)

Added Files:
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    pgsql/src/pl/plperl:
        plperl.h (r1.1)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plperl/plperl.h?rev=1.1&content-type=text/x-cvsweb-markup)

Removed Files:
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    pgsql/src/pl/plperl:
        spi_internal.h
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plperl/spi_internal.h)