ssh: introduce ini setting 'ssh_enabled', disabled by default

Administrators should control the use of SSH and may want to disable SSH
access, temporarily or permanently.

An explicit setting ssh_enabled is better than e.g. checking for a valid
ssh_authorized_keys setting, to allow such trivial temporary disabling.

To keep the controllers simple, introduce a decorator IfSshEnabled instead
of repeating the same config checks in every method.