ud-replicate, ud-generate: Instead of one big ssh-rsa-shadow file ud-generate

ud-replicate, ud-generate: Instead of one big ssh-rsa-shadow file ud-generate
now produces per-user authorized_keys files and tars them up.  On the receiving
end ud-replicate takes the tar and syncs it to userkeys/.  The goal here is to
no longer require a patched sshd.  Setting AuthorizedKeysFile2 to
/var/lib/misc/userkeys/%u is sufficient.  For homedir creation we can use
pam_mkhomedir. [mhy, sgran]