Cygwin: posix_spawnp: don't fallback to sh

Per the discussion starting with
https://cygwin.com/pipermail/cygwin/2023-April/253495.html
stop falling back to sh if the file given to posix_spawnp
is no executable.

This is not necessarily the last word on it, given
https://www.austingroupbugs.net/view.php?id=1674, but for now,
opt for following the proposal in the Austin Group bug entry,
as well as PASSing the GNULIB test-posix_spawnp-script test.

Fixes: c7c1a1ca1b ("Add support for new posix_spawn function.")
Fixes: 3fbfcd11fb ("Cygwin: posix_spawn: add Cygwin-specific code fixing process synchronisation")
Reported-by: Bruno Haible <bruno@clisp.org>
Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/cygwin/release/3.5.0

@@ -26,3 +26,10 @@ What's new:
 
 - New API calls: posix_spawn_file_actions_addchdir_np,
   posix_spawn_file_actions_addfchdir_np.
+
+What changed:
+-------------
+
+- posix_spawnp no longer falls back to starting the shell for unrecognized
+  files as execvp.  For the reasoning, see
+  https://www.austingroupbugs.net/view.php?id=1674

winsup/cygwin/spawn.cc

@@ -1392,8 +1392,7 @@ __posix_spawn_execvpe (const char *path, char * const *argv, char *const *envp,
   ch_spawn.set_sem (sem);
   ch_spawn.worker (use_env_path ? (find_exec (path, buf, "PATH", FE_NNF) ?: "")
 				: path,
-		   argv, envp,
-		   _P_OVERLAY | (use_env_path ? _P_PATH_TYPE_EXEC : 0));
+		   argv, envp, _P_OVERLAY);
   __posix_spawn_sem_release (sem, errno);
   return -1;
 }