* 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/cygwin.din

@@ -1144,14 +1144,14 @@ _tzname DATA
 ptsname
 grantpt
 unlockpt
-sexecve
-sexecl
-sexecle
-sexeclp
-sexeclpe
-sexecv
-sexecp
-sexecvpe
+sexecve = sexecve_is_bad
+sexecl = sexecve_is_bad
+sexecle = sexecve_is_bad
+sexeclp = sexecve_is_bad
+sexeclpe = sexecve_is_bad
+sexecv = sexecve_is_bad
+sexecp = sexecve_is_bad
+sexecvpe = sexecve_is_bad
 ttyslot
 rcmd = cygwin_rcmd
 rresvport = cygwin_rresvport