In use cases where one user is supposed to be reachable with multiple
public keys, but where each public key should only have access to a
specific set of commands, the variable $SSH_USER_AUTH will be considered
together with colon separated username->key pairs in the configuration
to determine the set of commands to use.
Signed-off-by: Georg Pfuetzenreuter <mail@georg-pfuetzenreuter.net>