sshproxy: check dump configuration settings on startup, and fail if incorrect

[dolmen]

Validate dump target settings on startup:

• if target is an output directory, check that the directory exists and is writable
• if target is a remote server, try connecting to it

If a check fails, abort startup immediately: the SSH session must be rejected.

[dolmen]

In cmd/sshproxy/recoder.go, NewRecorder should do checks and return an error if any.