Log Message:
fdm: Adjust mremap(2) usage on NetBSD

MREMAP_MAYMOVE flag is the default behaviour on NetBSD and by
adjusting the single mremap() call it can be used on NetBSD too
(remove CONFIGURE_ENV injection kludge).

Thanks to <joerg> and <kamil> respectively for kindly pointing out
that and suggestions! (possible regressions are mine!)