Use PerlLIO_dup_cloexec in Perl_dirp_dup to set O_CLOEXEC

dup doesn't mark the new descriptor as close-on-exec, which can lead to
a descriptor leaking to the new process.

(cherry picked from commit d6f09a8)

Author: Leont

sv.c

@@ -13959,7 +13959,7 @@ Perl_dirp_dup(pTHX_ DIR *const dp, CLONE_PARAMS *const param)
 
     PERL_UNUSED_ARG(param);
 
-    ret = fdopendir(dup(my_dirfd(dp)));
+    ret = fdopendir(PerlLIO_dup_cloexec(my_dirfd(dp)));
 
 #elif defined(WIN32)
     ret = win32_dirp_dup(dp, param);