Import selfpipe code from skalibs 1.2.5

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

third/libstddjb/selfpipe_internal.c

@@ -0,0 +1,31 @@
+/* ISC license. */
+
+/* MT-unsafe */
+
+#include <signal.h>
+#include "sysdeps.h"
+#include "selfpipe-internal.h"
+
+sigset_t selfpipe_caught ;
+
+#ifdef HASSIGNALFD
+
+int selfpipe_fd = -1 ;
+
+#else
+
+#include <errno.h>
+#include "allreadwrite.h"
+#include "djbunix.h"
+
+int selfpipe[2] = { -1, -1 } ;
+
+static void selfpipe_trigger (int s)
+{
+  char c = (char)s ;
+  fd_write(selfpipe[1], &c, 1) ;
+}
+
+struct skasigaction const selfpipe_ssa = { &selfpipe_trigger, SKASA_NOCLDSTOP | SKASA_MASKALL } ;
+
+#endif