fix(tmpfiles): Create /etc/ssh directory before its contents
fix(tmpfiles): Remove PAM related rules from /usr images.
fix(configs): Remove PAM and cleanup sshd config for usr

Remove any options that match system defaults or were only applicable to
using PAM. Add UsePrivilegeSeparation which is usually enabled in Gentoo.
Merge pull request #66 from marineam/usr

PXE fixes, PAM cleanup