ls: plug a per-argument leak

ls: plug a per-argument leak

Using ls -l on an SELinux-enabled system would leak one SELinux
context string per non-empty-directory command-line argument.
* src/ls.c (free_ent): New function, factored out of...
(clear_files): ...here.  Use it.
(extract_dirs_from_files): Call free_ent (f), rather than simply
free (f->name).  The latter failed to free the possibly-malloc'd
linkname and scontext members, and thus could leak one of those
strings per command-line argument.
* THANKS.in: Update.
* NEWS (Bug fixes): Mention it.
Reported by Juraj Marko in http://bugzilla.redhat.com/751974.