Rethink the generation rule for fmgroids.h macros.

Rethink the generation rule for fmgroids.h macros.

Traditionally, the names of fmgroids.h macros for pg_proc OIDs
have been constructed from the prosrc field.  But sometimes the
same C function underlies multiple pg_proc entries, forcing us
to make an arbitrary choice of which OID to reference; the other
entries are then not namable via fmgroids.h.  Moreover, we could
not have macros at all for pg_proc entries that aren't for
C-coded functions.

Instead, use the proname field, and append the proargtypes field
(replacing inter-argument spaces with underscores) if proname is
not unique.  Special-casing unique entries such as F_OIDEQ removes
the need to change a lot of code.  Indeed, I can only find two
places in the tree that need to be adjusted; while this changes
quite a few existing entries in fmgroids.h, few of them are
referenced from C code.

With this patch, all entries in pg_proc.dat have macros in fmgroids.h.

Discussion: https://postgr.es/m/472274.1604258384@sss.pgh.pa.us