Comprehensive spelling/typo fixes.

Thanks to Josh Soref for generating these fixes.
diff --git a/src/util.c b/src/util.c
index 53b4579..987a259 100644
--- a/src/util.c
+++ b/src/util.c
@@ -373,7 +373,7 @@
   return 0;
 }
 
-/* return least signigicant 64 bits if IPv6 address */
+/* return least significant 64 bits if IPv6 address */
 u64 addr6part(struct in6_addr *addr)
 {
   int i;