rename-tests: new test, exposes several platform bugs

rename-tests: new test, exposes several platform bugs

This test passes on GNU/Linux, OpenBSD, and Cygwin 1.7.
Elsewhere, this test fails because of at least these bugs:
Solaris 10, cygwin 1.5.x, and mingw all mistakenly succeed on
rename("file","other/").  Solaris 9 and the gnulib replacement
for SunOS 4.1 mistakenly succeed on rename("file/","other").
Cygwin 1.5.x and mingw mistakenly succeed on rename("dir","d/.").
Cygwin 1.5.x and NetBSD 1.6 (even with the gnulib replacement)
mistakenly reduce the link count on rename("hard1","hard2").

* modules/rename-tests: New file.
* tests/test-rename.h: Likewise.
* tests/test-rename.c: Likewise.
* doc/posix-functions/rename.texi (rename): Improve documentation,
including bugs that will eventually be fixed in gnulib.

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