Fixes to packages, new package for btrfs-progs

Updated a bunch of packages:
  - e2fsprogs
  - lxc
  - debootstrap
  - postfix was using /etc/aliases and /etc/postfix/virtual, so I changed those to /var/postfix/{virtual,aliases} because otherwise it doesn't work on read-only root
  - dovecot: the datadir was not being created
  - add package for btrfs-tools
  - disable suid in openssh and lxc