After 28000 attempts to bruteforce ftp connections last night
I have made ftpd a little less friendly.

1/ Drop the connection after 5 auth failures.

2/ Slow up the startup if we previously had an auth failure on this IP address.
the delay is exponential (to make them suffer) and is reset as soon as
we get a valid authenticated (I.E. not none or anonymous) connection from
this IP.

You will need to create /ftp/tmp with 0777 mode (and chmod +t it) to
enable latter part.

-Steve