chown: work around OpenBSD bug

chown: work around OpenBSD bug

chown(name,geteuid(),-1) failed to update the change time if
name was already owned by the current effective user.  Work
around it by using chmod, which does not have this bug.

Unfortunately, lchown has the same bug, but OpenBSD 4.0 lacks
lchmod and lutimes, so there is no way to affect ctime without
unlinking and recreating the symlink, which is too dangerous.

* lib/chown.c (rpl_chown): Work around the bug.
* lib/lchown.c (rpl_lchown): Attempt to do likewise.
* m4/chown.m4 (gl_FUNC_CHOWN): Test for ctime bug.
* m4/lchown.m4 (gl_FUNC_LCHOWN): Check for lchmod.
* modules/chown (Depends-on): Add stdbool.
* modules/lchown (Depends-on): Likewise.
* doc/posix-functions/chown.texi (chown): Document the bug.
* doc/posix-functions/lchown.texi (lchown): Likewise.
* tests/test-lchown.h (test_chown): Relax test.

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