Use isdirsep rather than SLASH_P throughout.

* path.cc (iscygdrive): Disallow /cygdrive\x.
(normalize_posix_path): "Normalize" a windows path, if detected, rather than
converting to posix.
* fhandler_serial.cc (fhandler_serial::tcsetattr): Add support and capability
checking for B230400 bitrate.
(fhandler_serial::tcgetattr): Add support for B230400 bitrate.
* include/sys/termios.h: Add B230400 definition for Posix support of 230.4Kbps.

winsup/cygwin/fhandler_proc.cc

@@ -87,7 +87,7 @@ fhandler_proc::get_proc_fhandler (const char *path)
    * it being normalised and therefore the path may have runs of slashes
    * in it.
    */
-  while (SLASH_P (*path))
+  while (isdirsep (*path))
     path++;
 
   /* Check if this is the root of the virtual filesystem (i.e. /proc).  */
@@ -105,7 +105,7 @@ fhandler_proc::get_proc_fhandler (const char *path)
 
   bool has_subdir = false;
   while (*path)
-    if (SLASH_P (*path++))
+    if (isdirsep (*path++))
       {
 	has_subdir = true;
 	break;