Add nonce plugin implementation

This nonce generator uses an RNG to generate nonces. The RNG quality is
currently set to RNG_WEAK which is the same value used in IKE init.

The plugin is enabled and thus built by default.

src/libstrongswan/Makefile.am

@@ -210,6 +210,13 @@ if MONOLITHIC
 endif
 endif
 
+if USE_NONCE
+  SUBDIRS += plugins/nonce
+if MONOLITHIC
+  libstrongswan_la_LIBADD += plugins/nonce/libstrongswan-nonce.la
+endif
+endif
+
 if USE_HMAC
   SUBDIRS += plugins/hmac
 if MONOLITHIC