Now new_connection() takes an optional LogPolicy * argument, and
passes it on to the SshProxy setup. This means that SshProxy's
implementation of the LogPolicy trait can answer queries like
askappend() and logging_error() by passing them on to the same
LogPolicy used by the main backend.
Not all callers of new_connection have a LogPolicy, so we still have
to fall back to the previous conservative default behaviour if
SshProxy doesn't have a LogPolicy it can ask.
The main backend implementations didn't _quite_ have access to a
LogPolicy already, but they do have a LogContext, which has a
LogPolicy vtable pointer inside it; so I've added a query function
log_get_policy() which allows them to extract that pointer to pass to
new_connection.
This is the first step of fixing the non-interactivity limitations of
SshProxy. But it's also the easiest step: the next ones will be more
involved.
This clears up another large pile of clutter at the top level, and in
the process, allows me to rename source files to things that don't all
have that annoying 'ssh' prefix at the top.
