Naughty NANDFS was using hidden unused flag, hiding the fact that the
flag was used and wasn't really available. Change the name without
fixing any laying issues that might be present in NANDFS' use of this
flag.
Fix typo in flag name.
rm -rf can fail sometimes with an error from fts_read. Make it honor
fflag to ignore fts_read errors, but stop deleting from that directory
because no further progress can be made.

When building a kernel with a high -j value on a high core count
machine, during the cleanobj phase we can wind up doing multiple rm
-rf at the same time for modules that have subdirectories. This
exposed this race (sometimes) as fts_read can return an error if the
directory is removed by another rm -rf. Since the intent of the -f
flag was to ignore errors, even if this was a bug in fts_read, we
should ignore the error like we've been instructed to do.