lchown: detect Solaris and FreeBSD bug

lchown: detect Solaris and FreeBSD bug

Solaris 9 and FreeBSD 7.2 lchown("link-to-file/",uid,gid)
mistakenly changes ownership of "file".

* lib/lchown.c (rpl_lchown): Work around bug.
* m4/lchown.m4 (gl_FUNC_LCHOWN): Check for trailing slash bugs.
* m4/unistd_h.m4 (gl_UNISTD_H_DEFAULTS): Add witness.
* modules/unistd (Makefile.am): Populate it.
* lib/unistd.in.h (lchown): Update declaration.
* doc/posix-functions/lchown.texi (lchown): Document the bug.
* modules/lchown-tests: New file.
* tests/test-lchown.h (test_lchown): Likewise.
* tests/test-lchown.c (main): Likewise.

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