Move port-forwarding setup out of ssh.c.

The tree234 storing currently active port forwardings - both local and
remote - now lives in portfwd.c, as does the complicated function that
updates it based on a Conf listing the new set of desired forwardings.

Local port forwardings are passed to ssh.c via the same route as
before - once the listening port receives a connection and portfwd.c
knows where it should be directed to (in particular, after the SOCKS
exchange, if any), it calls ssh_send_port_open.

Remote forwardings are now initiated by calling ssh_rportfwd_alloc,
which adds an entry to the rportfwds tree (which _is_ still in ssh.c,
and still confusingly sorted by a different criterion depending on SSH
protocol version) and sends out the appropriate protocol request.
ssh_rportfwd_remove cancels one again, sending a protocol request too.

Those functions look enough like ssh_{alloc,remove}_sharing_rportfwd
that I've merged those into the new pair as well - now allocating an
rportfwd allows you to specify either a destination host/port or a
sharing context, and returns a handy pointer you can use to cancel the
forwarding later.

misc.c

@@ -1181,6 +1181,17 @@ int smemeq(const void *av, const void *bv, size_t len)
     return (0x100 - val) >> 8;
 }
 
+int nullstrcmp(const char *a, const char *b)
+{
+    if (a == NULL && b == NULL)
+        return 0;
+    if (a == NULL)
+        return -1;
+    if (b == NULL)
+        return +1;
+    return strcmp(a, b);
+}
+
 ptrlen make_ptrlen(const void *ptr, size_t len)
 {
     ptrlen pl;