Fix SCP support, but break jail

[danielskowronski]

Fix SCP support, but break jail. Proposing this change anyway as I really needed support for SCP from an embedded system (some modern systems somehow issue SFTP even when scp client is called).

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Unfortunately, OpenSSH allows jailing with SFTP by using a dirty hack - internal-sftp is a magic way to call internal SFTP process, allowing user to break the rules of ChrootDirectory, which require a set of files to be under path specified (things like binary specified in ForceCommand and basic /dev nodes).

At the same time, internal-sftp binary (/usr/lib/openssh/sftp-server) completely delegates jailing/chrooting to OpenSSH and has no command-line options to restrict what it can read/write.

On the other hand, scp can be somewhat jailed on its own (except symlinks, which should be doable by other tools on Linux) due to a fixed way it sends and receives files with -f, -t and -d.

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Maybe some alternative SSH server should also be considered - I was originally hoping for the ability to Match selected subsystem and based on that set standard SFTP config or just leave ForceCommand with proxy script for SCP.