Add dropbox_port_t support

The dropbox application has a feature called "LAN Sync" which works on
TCP & UDP port 17500. Marking this port as dropbox_port_t (instead of
the currently default unreserved_port_t) allows for more fine-grained
access control to this resource.

Signed-off-by: Sven Vermeulen <sven.vermeulen@siphos.be>
Module version bump for dropbox port from Sven Vermeulen.