Use /dev/urandom

http://sockpuppet.org/blog/2014/02/25/safely-generate-random-numbers/
https://speakerdeck.com/filosottile/the-plain-simple-reality-of-entropy

Also remove the iflag=fullblock GNUism that doesn't exist on BSD and OSX

README.md

@@ -16,5 +16,5 @@ To build and install the latest version:
 
 To create and use a new secret key:
 
-    $ dd if=/dev/random iflag=fullblock of=glorytun.key bs=32 count=1
+    $ dd if=/dev/urandom of=glorytun.key bs=32 count=1
     # glorytun keyfile glorytun.key [...]