* environ.cc (set_winsymlinks): Handle "winsymlinks:nativestrict"

option.  On pre-Vista warn the user if the "winsymlinks:native*" option
	is set.
	* globals.cc (enum winsym_t): Add WSYM_nativestrict.
	* path.cc (symlink_native): Don't create native symlink if target
	does not exist.  Explain why.  Improve comments.
	(symlink_worker): Change AFS symlink handling to WSYM_nativestrict.
	Handle WSYM_nativestrict throughout.  Change condition for bail out
	to wsym_type == WSYM_nativestrict.  Add comment.  Fix formatting.
	* shared_info.h (CURR_USER_MAGIC): Change to reflect change in
	class user_info.
	(class user_info): Add member warned_nonativesyms.

winsup/cygwin/globals.cc

@@ -56,6 +56,7 @@ enum winsym_t
   WSYM_sysfile = 0,
   WSYM_lnk,
   WSYM_native,
+  WSYM_nativestrict,
   WSYM_nfs
 };