sftp: Be a bit more clever about completions

sftp: Be a bit more clever about completions

There are commands (e.g. "get" or "put") that accept two
arguments, a local path and a remote path. However, the way
current completion is written doesn't take this distinction into
account and always completes remote or local paths.

By expanding CMD struct and "cmds" array this distinction can be
reflected and with small adjustment to completer code the correct
path can be completed.

By Michal Privoznik, ok dtucker@