Move pqueue into ssl

This is an internal facility, never documented, not for
public consumption.  Move it into ssl (where it's only used
for DTLS).

I also made the typedef's for pqueue and pitem follow our style: they
name structures, not pointers.

Reviewed-by: Richard Levitte <levitte@openssl.org>

ssl/d1_lib.c

@@ -201,8 +201,8 @@ void dtls1_free(SSL *s)
 
 void dtls1_clear(SSL *s)
 {
-    pqueue buffered_messages;
-    pqueue sent_messages;
+    pqueue *buffered_messages;
+    pqueue *sent_messages;
     unsigned int mtu;
     unsigned int link_mtu;