rename-dest-slash: merge into rename module

rename-dest-slash: merge into rename module

Tested that NetBSD workaround still passes unit test.

* modules/rename-dest-slash (Status): Mark obsolete.
(Depends-on): Add rename.
(Files): Let rename do it all.
* m4/rename.m4 (gl_FUNC_RENAME): Also test for NetBSD bugs,
subsuming the test from gl_FUNC_RENAME_TRAILING_DEST_SLASH...
* m4/rename-dest-slash.m4: ...so this file can be deleted.
* lib/rename-dest-slash.c (rpl_rename_dest_slash): Delete.
* lib/rename.c (rpl_rename): Update comments.

Signed-off-by: Eric Blake <ebb9@byu.net>