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/Makefile b/Makefile
index 0ce4b32..f055a08 100644
--- a/Makefile
+++ b/Makefile
@@ -309,10 +309,11 @@
   USE_TPROXY     = implicit
 else
 ifeq ($(TARGET),openbsd)
-  # This is for OpenBSD >= 3.0
+  # This is for OpenBSD >= 5.7
   USE_POLL       = implicit
   USE_KQUEUE     = implicit
   USE_TPROXY     = implicit
+  USE_ACCEPT4    = implicit
 else
 ifeq ($(TARGET),netbsd)
   # This is for NetBSD
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