generating fmgr prototypes automatically

During some recent patch reviews, there was some back and forth about
where to put prototypes for fmgr-callable functions. We don't actually
need these prototypes except to satisfy the compiler, so we end up
sprinkling them around random header files.

I figured we could generate these prototypes automatically using the
existing Gen_fmgrtab.pl script. That ends up saving more than 2000
lines of manual code, and it helps detect when a function exists in code
but is not registered in pg_proc.h.

... which this actually found in cash.c. I ended up adding the missing
entries in pg_proc and pg_operator, but we could also opt to just remove
the unused code.

There are long-standing symbol clashes from the lo_* functions between
the backend and libpq. So far this hasn't bothered anyone, but because
this patch rearranges some header files, the libpqwalreceiver code gets
upset. So I renamed the C symbols for the backends functions in a
separate patch. The user-visible function names remain the same.

--
Peter Eisentraut http://www.2ndQuadrant.com/
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