Proposed dirmod.c fix for Win32

The current version of dirmod.c causes a compilation failure under MingW:

../../../src/port/libpgport.a(dirmod.o.b)(.text+0xe1): In function
`pgrename':
e:/cygwin/opt/diff8c/pgsql/src/port/dirmod.c:38: undefined reference to
`errstart'
../../../src/port/libpgport.a(dirmod.o.b)(.text+0xef):e:/cygwin/opt/diff8c/p
gsql/src/port/dirmod.c:38: undefined reference to `elog_finish'

and so on.

[dirmod.c provides replacements for unlink + rename under Win32. These
functions are currently only ever used by the backend code, and by
pg_resetxlog]

One solution is, obviously, to drop the elog calls...

A somewhat better solution is to move dirmod.c into
src/backend/port(/win32?), and just compile dirmod.c directly into the
backend. pg_resetxlog could then use dirmod.c just as it currently uses
pg_crc.c (refer to Makefile in pg_resetxlog directory). One additional
requirement would be for pg_resetxlog to be considered a FRONTEND component.

Does anyone take issue with this (and/or is it advisable to add -DFRONTEND
to pg_resetxlog Makefile; seems ok to me), or have a better idea, before I
go create/submit a patch?

Cheers,
Claudio

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