Bug#1006171: Make internal-sftp the default

I understand and support Colin's stance that the default configuration 
shipped with Debian should follow upstream.

The nasty thing about subsystem directives is that they cannot be 
overridden by a .conf file placed inside the /etc/ssh/sshd_config.d/ 
folder, due to this bug: 
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=998834

So if one wants to use e.g. the internal sftp server, one MUST modify 
the /etc/ssh/sshd_config file. This in turn interferes with automatic 
upgrade scripts like cron-apt, unattended-upgrades etc.

I thought about moving the subsystem directive into a new file inside 
the /etc/ssh/sshd_config.d/ folder. However, this would result in broken 
configurations on machines where the administrator keeps a modified 
/etc/ssh/sshd_config file during an upgrade.