Import selfpipe code from skalibs 1.2.5

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

third/libstddjb/fd_write.c

@@ -0,0 +1,13 @@
+/* ISC license. */
+
+#include <unistd.h>
+#include <errno.h>
+#include "allreadwrite.h"
+
+int fd_write (int fd, char const *buf, unsigned int len)
+{
+  register int r ;
+  do r = write(fd, buf, len) ;
+  while ((r == -1) && (errno == EINTR)) ;
+  return r ;
+}