Implemented dump_print_message. Fixes #9

This bug was occurring because going into dump mode did not replace the curses version of print_message when it replaced the other related windowport methods, so stuff intended for the dumpfile was winding up in the status line.  I've fixed that by implementing dump_print_message, dump_print_message_core, and dump_print_message_nonblocking, which simply write the line out to the dumpfile.