mod_sftp should fail on startup when SFTP and TLS are both enabled for a vhost

[Castaglia]

I have seen users' configurations where they are not using different <VirtualHost> sections for FTP/FTPS vs SFTP servers, and instead have e.g.:

<IfModule mod_tls.c>
  TLSEngine on
  ...
</IfModule>

<IfModule mod_sftp.c>
    SFTPEngine on
    ...
</IfModule>

This lets administrators think that they can support FTP/FTPS and SFTP using the same single vhost -- but they cannot. The protocols do not work that way.

So the scope of this task is to add detection of this sort of configuration to the mod_sftp module (since it is a contrib module, not a core module), and to log a warning to the system log when this happens. Or perhaps mod_sftp should check for such configurations on startup, and prevent the server from passing its config check? That would be better...