fts: add/use new struct member, fts_dirp

fts: add/use new struct member, fts_dirp

We are about to use this to manage any directory with
too many entries to read all of them into memory at once.
To do that, we'll need to save the DIR* pointer in each
affected FTSENT struct.
* lib/fts_.h: Include <dirent.h>.
(struct FTSENT) [fts_dirp]: New member.
* lib/fts.c (closedir_and_clear): Define.
Use it in place of closedir so that we are sure to
clear the new fts_dirp member when done with it.
(fts_alloc): Initialize the new member.
(fts_lfree): Free, if needed.