BUILD: Make use of accept4() on OpenBSD.

OpenBSD >= 5.7 supports accept4(). Older versions are not supported
anymore anyway.

Patch originally from Brad Smith.
diff --git a/README b/README
index 16982e1..074a3b0 100644
--- a/README
+++ b/README
@@ -43,7 +43,7 @@
   - freebsd     for FreeBSD 5 to 10 (others untested)
   - netbsd      for NetBSD
   - osx         for Mac OS/X
-  - openbsd     for OpenBSD 3.1 and above
+  - openbsd     for OpenBSD 5.7 and above
   - aix51       for AIX 5.1
   - aix52       for AIX 5.2
   - cygwin      for Cygwin