SSH/LDAP migration

Why migrate?
Originally, Gentoo used OpenSSH LDAP public key patch from Eric Auge. However, this patch is dead and doesn't work anymore with OpenSSH 7.7 or newer because auth_parse_options function was removed in OpenSSH via commit 7c856857607112a3dfe6414696bf4c7ab7fb0cb3.

In the meantime this patch was rewritten by Red Hat from scratch. Instead of patching OpenSSH, the new implementation make use of OpenSSH's AuthorizedKeysCommand feature. However, this requires a configuration change.

...but someone needs to write that guide first.