* mk-sbuild: conditionally install lvm2 only for lvm-snapshot schroots
* mk-sbuild: rationalise architecture variables
* mk-sbuild: Generalise --type support and add "file" SCHROOT_TYPE
* mk-sbuild.1: Document the --type argument
* mk-sbuild: clean up and make slight adjustments to new lvm/dir/file logic.
* mk-sbuild: clean up and make slight adjustments to new lvm/aufs logic.
* mk-sbuild.1: update documentation to reflect alternative config file
names for distro and schroot type overrides.