libc: Check for readdir(2) errors in fts(3)

Previously, such errors were not distinguished from the end-of-directory
condition.

With improvements from Mahmoud Abumandour <ma.mandourr@gmail.com>.

Reviewed by: markj
PR: 262038

(cherry picked from commit 0cff70ca66547ca5b04030ef07e6a0b9759a0184)