Make tlsext_tick_lifetime_hint an unsigned long (from signed long).

From RFC4507:
"The ticket_lifetime_hint field contains a hint from the server about how
long the ticket should be stored.  The value indicates the lifetime in
seconds as a 32-bit unsigned integer in network byte order."

Reviewed-by: Tim Hudson <tjh@openssl.org>

ssl/ssl_locl.h

@@ -670,7 +670,7 @@ struct ssl_session_st {
     /* RFC4507 info */
     unsigned char *tlsext_tick; /* Session ticket */
     size_t tlsext_ticklen;      /* Session ticket length */
-    long tlsext_tick_lifetime_hint; /* Session lifetime hint in seconds */
+    unsigned long tlsext_tick_lifetime_hint; /* Session lifetime hint in seconds */
 # endif
 # ifndef OPENSSL_NO_SRP
     char *srp_username;