Use a fd opened for read/write when syncing slots during startup, take 2.

Use a fd opened for read/write when syncing slots during startup, take 2.

Cribbing from dfbaed45975:
    Some operating systems, including the reporter's windows, return EBADFD
    or similar when fsync() is invoked on a O_RDONLY file descriptor.
    Unfortunately RestoreSlotFromDisk() does exactly that; which causes
    failures after restarts in at least some scenarios.

    If you hit the bug the error message will be something like
    ERROR: could not fsync file "pg_replslot/$name/state": Bad file descriptor

    Simply use O_RDWR instead of O_RDONLY when opening the relevant file
    descriptor to fix the bug.

Unfortunately this fix was undone in 82a5649fb9db. Re-apply, and add a
comment.

Bug: 16039
Reported-By: Hans Buschmann
Author: Andres Freund
Discussion: https://postgr.es/m/16039-196fc97cc05e141c@postgresql.org
Backpatch: 12-, as 82a5649fb9db