priv-set: new module and accompanying tests; adapt write-any-file

priv-set: new module and accompanying tests; adapt write-any-file

* lib/priv-set.c: New file.
* lib/priv-set.h: New file.
* lib/unlinkdir.c: Make cannot_unlink_dir thread-safe.
* lib/write-any-file.c: Simplify by using priv-set module.
* m4/priv-set.m4: New file.
* modules/priv-set: New file.
* modules/unlinkdir: Add dependency on priv-set module.
* modules/write-any-file: Likewise.
These changes address a problem that originally arose with GNU tar:
http://thread.gmane.org/gmane.comp.gnu.tar.bugs/3242

Tests for module 'priv-set'.
* modules/priv-set-tests: New file.