mv, rm: adapt to new and improved gnulib interfaces

mv, rm: adapt to new and improved gnulib interfaces

Use gnulib's new priv-set module and updated write-any-file.
With them, the remove-called can_write_any_file function no
longer tries to drop the unlink-directory privilege, so now
each caller of remove must do that separately, calling
priv_set_remove_linkdir.
* bootstrap.conf (gnulib_modules): Add priv-set.
* src/rm.c: Include "priv-set.h".
(main): Call priv_set_remove_linkdir.
* src/mv.c (main): Likewise.
* gnulib: Update submodule to latest.