Fix various little bugs and loosen up various warnings

Change-Id: Iead48ac030290290c7c448b6f72a31380c2e8326
diff --git a/third_party/libevent/evutil_rand.c b/third_party/libevent/evutil_rand.c
index 284341c..8641f3d 100644
--- a/third_party/libevent/evutil_rand.c
+++ b/third_party/libevent/evutil_rand.c
@@ -174,7 +174,7 @@
 void
 evutil_secure_rng_add_bytes(const char *buf, size_t n)
 {
-	arc4random_addrandom((unsigned char*)buf,
+	arc4random_addrandom((const unsigned char*)buf,
 	    n>(size_t)INT_MAX ? INT_MAX : (int)n);
 }