fcm make: extract: support SSH location

This change allows extract from file system locations on a remote host accessible by SSH and RSYNC. I have rationalised some settings:
* `mirror.prop{ssh.pwd}` and `mirror.prop{ssh.mkdir}` are removed. It is very unlikely that they are going to be used.
* The defaults commands and options for `ssh` and `rsync` are now in `FCM::Util` and the site/user external configuration. (They would still be configurable by `mirror{prop}`.)

Close #13.