juju scp should accept multiple targets and pass extra args to scp

OpenSSH scp command supports multiple source and destination targets, like:

scp user1@host1:/path/file1 user@host2:/path/file2 local-dir/

Juju scp command only parses the first and second argument and interprets them as source/destination. If should miminc scp more closely by accepting multiple remote and/or local paths, so that the above can be called as:

$ juju scp ubuntu/0:/var/log/juju/unit-ubuntu-0.log 0:/var/log/juju/all-machines.log remote-logs/
(assuming we have machine 0 and unit ubuntu/0 deployed on machine 1, and the local remote-logs dir exists)

The expected result is the command to succeed and both logs should appear in remote-logs/, but instead in trunk the result is:

ERROR exit status 1 (Warning: Permanently added 'ec2-54-228-163-187.eu-west-1.compute.amazonaws.com,54.228.163.187' (ECDSA) to the list of known hosts.
Host key verification failed.
lost connection)

Also, even though juju scp's help says it supports passing extra arguments to scp, by giving them after --, like:

juju scp -- -r mongodb/0:/var/log/mongodb/ remote-logs/

In fact the implementation does not parse or forward any extra arguments in a way scp can understand:

$ juju scp -- -r 0:/var/log/juju remote-logs/

The expected result is scp succeeds and recursively copies the /var/log/juju dir to remote-logs/, but instead:

ERROR exit status 1 (usage: scp [-12346BCpqrv] [-c cipher] [-F ssh_config] [-i identity_file]
           [-l limit] [-o ssh_option] [-P port] [-S program]
           [[user@]host1:]file1 ... [[user@]host2:]file2)