clarify license and origin for pidwraptest

The original author clarified the license, so we are fine to ship with
the PID wraparound test. Run it by default if libressl is using the
native arc4random supplied from the OS.

ok deraadt@

tests/Makefile.am.tpl

@@ -6,3 +6,7 @@ AM_CPPFLAGS += -I $(top_srcdir)/crypto/asn1
 TESTS =
 check_PROGRAMS =
 EXTRA_DIST =
+
+if !NO_ARC4RANDOM_BUF
+TESTS += pidwraptest.sh
+endif

tests/pidwraptest.c

@@ -1,6 +1,9 @@
 /*
- * Checks if LibreSSL's PRNG is fork-safe on Linux.
+ * Checks if LibreSSL's PRNG is fork-safe.
  * From https://www.agwa.name/blog/post/libressls_prng_is_unsafe_on_linux
+ * This code is in the public domain.
+ *
+ * Original source: https://gist.github.com/AGWA/eb84e55ca25a7da1deb0
  */
 
 #include <openssl/rand.h>