CLOEXEC: use open(CLOEXEC) if exist; do not check fcntl(FD_CLOEXEC) failure

CLOEXEC: use open(CLOEXEC) if exist; do not check fcntl(FD_CLOEXEC) failure

    text           data     bss     dec     hex filename
-    370              0       0     370     172 libc/misc/dirent/opendir.o
+    366              0       0     366     16e libc/misc/dirent/opendir.o
-    375              4       0     379     17b libc/pwd_grp/lckpwdf.o
+    356              4       0     360     168 libc/pwd_grp/lckpwdf.o
-    248              0       0     248      f8 librt/shm.o
+    209              0       0     209      d1 librt/shm.o

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>