SH_DENYRW (deny-all) locks were broken 18 years ago with commit b866f199.

This fixes that by treating SH_COMPAT the same as SH_DENYNO (only), rather
than the same as SH_DENYNO or SH_DENYRW (and performing no locking).

This is likely the/a root-cause of some SBBS file corruption on *nix systems.