Import selfpipe code from skalibs 1.2.5

[andersk@mit.edu: Move code to new third/libstddjb directory]

third/libstddjb/coe.c

@@ -0,0 +1,11 @@
+/* ISC license. */
+
+#include <fcntl.h>
+#include "djbunix.h"
+
+int coe (int fd)
+{
+   register int flags = fcntl(fd, F_GETFD, 0) ;
+   if (flags < 0) return -1 ;
+   return fcntl(fd, F_SETFD, flags | FD_CLOEXEC) ;
+}