transaction: check for conflicts, not supersets, when gauging destructiveness
core: move away control-request unit file cmd handlers to ugly and repetitive util functions
control-request: add create-snapshot
manager: don't add to D-Bus queue when preparing a unit load
control-request: add (buggy) remove-snapshot
core: place arg files as macros in special.h
core: centralize unit file commands into a clean function unit_file_operation_tango()
control-request: add scaffolding for cancel-job