* external.cc (cygwin_internal): Export __cygwin_user_data.

* include/sys/cygwin.h: Allow definition of per_process even when not compiling
with C++.
(cygwin_getinfo_types): Add CW_USER_DATA.
* lib/_cygwin_crt0_common.cc (_cygwin_crt0_common): Get __cygwin_user_data
pointer from cygwin_internal.  If it doesn't exist, return failure.  Use either
this pointer or passed in pointer throughout.  Clear forkee.
* lib/crt0.h: Accomodate argument changes to _cygwin_crt0_common.
* lib/cygwin_attach_dll.c (cygwin_attach_dll): Reorganize to allow use of newer
binaries with older DLLs.  Detect older DLLs when _cygwin_crt0_common returns 0
and allocate space for a per_process structure on the stack.
* lib/cygwin_crt0.c (cygwin_crt0): Ditto.

winsup/cygwin/include/sys/cygwin.h

@@ -36,7 +36,6 @@ extern void cygwin_premain1 (int argc, char **argv);
 extern void cygwin_premain2 (int argc, char **argv);
 extern void cygwin_premain3 (int argc, char **argv);
 
-#ifdef __cplusplus
 /* This lives in the app and is initialized before jumping into the DLL.
    It should only contain stuff which the user's process needs to see, or
    which is needed before the user pointer is initialized, or is needed to
@@ -126,7 +125,6 @@ struct per_process
   struct _reent *impure_ptr;
 };
 #define per_process_overwrite ((unsigned) &(((struct per_process *) NULL)->resourcelocks))
-#endif /* __cplusplus */
 
 #ifdef _PATH_PASSWD
 extern HANDLE cygwin_logon_user (const struct passwd *, const char *);
@@ -152,7 +150,8 @@ typedef enum
     CW_SETPINFO,
     CW_SETTHREADNAME,
     CW_GETVERSIONINFO,
-    CW_READ_V1_MOUNT_TABLES
+    CW_READ_V1_MOUNT_TABLES,
+    CW_USER_DATA
   } cygwin_getinfo_types;
 
 struct external_pinfo