Don't check directories always case-insensitive in rename(2)

Long-standing problem in one of the corner cases of rename(2):
If we rename a directory a check is performed to see if newpath is
identical to oldpath or a subdir of oldpath. This check is
(accidentally? no hints anywhere in ChangeLogs or code) performed
case-insensitive for as long as we use Unicode paths and NT functions.
This leads to the problems described in
https://cygwin.com/ml/cygwin/2016-09/msg00264.html

Change this to be conditional case-sensitive as all other checks but
let's take this with a grain of salt.  There may be corner-cases in
this corner-case which require to chek parts of the path always
case-insensitive.  Off the top of my head I can't construct such a
case but that's no proof they don't exist :}

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/cygwin/release/2.6.1

@@ -11,3 +11,6 @@ Bug Fixes
 
 - Fix regression in console charset handling
   Addresses: https://cygwin.com/ml/cygwin/2016-10/msg00000.html
+
+- Fix case-sensitivity problem when renaming directories
+  Addresses: https://cygwin.com/ml/cygwin/2016-09/msg00264.html

winsup/cygwin/syscalls.cc

@@ -2224,7 +2224,7 @@ rename (const char *oldpath, const char *newpath)
 	  /* Check for newpath being identical or a subdir of oldpath. */
 	  if (RtlPrefixUnicodeString (oldpc.get_nt_native_path (),
 				      newpc.get_nt_native_path (),
-				      TRUE))
+				      oldpc.objcaseinsensitive ()))
 	    {
 	      if (newpc.get_nt_native_path ()->Length
 		  == oldpc.get_nt_native_path ()->Length)