v1.20 2015 Sept 24th p1

Really fixed URL backward compatibility (use -url_backward_compatibility=true if you have a wdt v20 talking to v18 or earlier)

new -exit_on_bad_flags=false if you want to continue despite bad command line (useful for forward compatibility)

shorter urls (wdt://host:port?num_port=4) and correct escaping of ipv6 [addr]

wdt receiver will now not overwrite any file by default unless you add -overwrite or use download resumption

misc bug fixes and cleanup