support for non-system users

[Igor Bukanov]

Hello,

In my setup I use ssh running on the host to login into different
containers. As users in the containers are not known to the host, to
support that I use a dummy user per container on the host with a database
that maps public keys into container users. Then I use the `command` option
for authorized keys to invoke a command that connects to the container
where one of the arguments is the username inferred from the key.

This works (and thanks for recent changes to support AuthorizedKeysCommand
custom arguments made things even simpler!), but it has few drawbacks.

As I want to allow to use the same public key to login to different
containers, I cannot use `root` for a dummy user. But since operations with
containers are privileged, I have to use setuid/setguid command to
eventually start a process in the container from the process under a dummy
user account on host. Another problem is that those dummy users should be
created on the host. This complicates container setup.

So would it be possible to add an options to sshd_command to run
ForceCommand under a different user than the one that tries to login? This
will resolve the first concern. Another option would be to allow to use
non-system user names with sshd trusting that AuthorizedKeysCommand and
ForceCommand would do the proper job with user verification.