Import selfpipe code from skalibs 1.2.5

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

third/libstddjb/sanitize_read.c

@@ -0,0 +1,15 @@
+/* ISC license. */
+
+#include <errno.h>
+#include "allreadwrite.h"
+#include "error.h"
+
+int sanitize_read (int r)
+{
+  switch (r)
+  {
+    case -1 : return error_isagain(errno) ? (errno = 0, 0) : -1 ;
+    case 0  : return (errno = EPIPE, -1) ;
+    default : return r ;
+  }
+}