This commit adds a new 'known-hosts2' field to the OpenSSH home service
configuration, enabling a hybrid approach to SSH host key management.
* gnu/home/services/ssh.scm (<home-openssh-configuration>)[known-hosts2]: New field.
(openssh-configuration-files): Generate ~/.ssh/known_hosts2 when specified.
* doc/guix.texi (Secure Shell): Document new 'known-hosts2' field.
Change-Id: I1d314706eaf6af9547833020abe857f4d8c44b86