* Makefile.in: Ensure that -MD gets added to CFLAGS regardless of CFLAGS

command-line setting.
* cygwin.din: Export sexec* functions as function which returns ENOSYS
(i.e., sexec* is deprecated).
* dtable.cc (dtable::vfork_child_dup): Ensure that impersonation is restored
even on failure.
* exec.cc: Throughout, remove references to sexec* and _spawnve.
* pinfo.h: Remove _spawnve declaration.
* spawn.cc: Rename _spawnve to spawnve and use throughout.
(spawn_guts): Eliminate hToken argument and processing of same.  Just perform
special actions if impersonating.
(spawnve): Rename from _spawnve.

winsup/cygwin/pinfo.h

@@ -199,9 +199,6 @@ void __stdcall set_myself (pid_t pid, HANDLE h = NULL);
 extern pinfo myself;
 
 #define _P_VFORK 0
-extern "C" int _spawnve (HANDLE hToken, int mode, const char *path,
-			 const char *const *argv, const char *const *envp);
-
 extern void __stdcall pinfo_fixup_after_fork ();
 extern HANDLE hexec_proc;