pgsql: Fix Gen_fmgrtab.sh to not rely on hard-wired knowledge of the

Log Message:
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Fix Gen_fmgrtab.sh to not rely on hard-wired knowledge of the column numbers
in pg_proc. Also make it not emit duplicate extern declarations, and make it
a bit more bulletproof in some other small ways. Likewise fix the equally
hard-wired, and utterly undocumented, knowledge in the MSVC build scripts.
For testing purposes and perhaps other uses in future, pull out that portion
of the MSVC scripts into a standalone perl script equivalent to
Gen_fmgrtab.sh, and make it generate actually identical output, rather than
just more-or-less-the-same output.

Motivated by looking at Pavel's variadic function patch. Whether or not
that gets accepted, we can be sure that pg_proc's column set will change
again in the future; it's time to not have to deal with this gotcha.

Modified Files:
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pgsql/src/backend/utils:
Gen_fmgrtab.sh (r1.39 -> r1.40)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/Gen_fmgrtab.sh?r1=1.39&r2=1.40)
pgsql/src/tools/msvc:
Solution.pm (r1.41 -> r1.42)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/tools/msvc/Solution.pm?r1=1.41&r2=1.42)

Added Files:
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pgsql/src/backend/utils:
Gen_fmgrtab.pl (r1.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/Gen_fmgrtab.pl?rev=1.1&content-type=text/x-cvsweb-markup)