1. Add a copyright at the top.
2. Make passive and binary modes work with new ftpio semantics.
3. Add a `mirror' (-m) flag which only copies the remote file if it's
not the same size or is newer.
1. Always use file modtime, and if that's not usable then default
to current time.
2. Allow HTTP_TIMEOUT to be set as an environment variable to override
default.
3. Handle SIGTERM.
