Cygwin: change virtual_ftype_t to not rely on negative values

So far negative values were denoting files, positive values
denoting directories.  We should prefer a less error prone
method.  Redefine virtual_ftype_t to contain only positive
values and replace checks for negativ or positive values with
inline functions virt_ftype_isfile() and virt_ftype_isdir().

Drop outdcated comments referring to numerical virtual_ftype_t
values.

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

winsup/cygwin/fhandler_process.cc

@@ -169,7 +169,7 @@ fhandler_process::fstat (struct stat *buf)
       buf->st_uid = p->uid;
       buf->st_gid = p->gid;
       buf->st_mode |= S_IFDIR | S_IXUSR | S_IXGRP | S_IXOTH;
-      if (file_type == 1)
+      if (file_type == virt_directory)
 	buf->st_nlink = 2;
       else
 	buf->st_nlink = 3;