Don't complain if do_builtin_io fails due to EPIPE

If stdio is dead due to EPIPE, there's no great reason to spew a stack dump.

This will still write an error to stderr if stdout dies.  This might be
undesirable, but changing that should be considered separately.

src/postfork.cpp

@@ -569,12 +569,13 @@ bool do_builtin_io(const char *out, size_t outlen, const char *err, size_t errle
     bool success = true;
     if (out && outlen)
     {
-
         if (write_loop(STDOUT_FILENO, out, outlen) < 0)
         {
+            int e = errno;
             debug_safe(0, "Error while writing to stdout");
             safe_perror("write_loop");
             success = false;
+            errno = e;
         }
     }