add .ssh/authorized_keys support for --sshcopyid

add .ssh/authorized_keys support for --sshcopyid

We retain backwards compatibility, that is: we use keys from the agent
by default. But if unavailable, we tap into the ~/.ssh/authorized_keys
file (or whatever is specified by the $AUTHORIZED_KEYS_SOURCE
environment).

The target SSH directory can be changed with $AUTHORIZED_KEYS_TARGET.

Closes: #151