tos3to4: Migrate sshd config

TOS3 used /etc/config/sshd while TOS5 uses /etc/ssh/sshd_config for OpenSSH server configuration. Without migration, any user modifications get lost which can lead to the router becoming unreachable via SSH (e.g., due to listening on the wrong port, using different auth settings, etc.).

IMHO the migration could be as easy as copying the existing file to the new location as both seem to be identical by default.