ls: check fts_children() for errors that may not surface otherwise

In particular, if one simply does a non-recursive ls on a directory
that is not accessible, there are some classes of errors that may cause
it to fail that wouldn't be surfaced unless we do an fts_read() that
will recurse into the inaccessible directory. Catch those kinds of
errors here since we cannot expect to an FTS_ERR/FTS_DNR entry to follow
up on them.

PR: 287451
Reviewed by: kib
Discusssed with: des
Differential Revision: https://reviews.freebsd.org/D51056