Reactivate OP_RUN

[nsoblath]

Note that this task is dependent on whether the proposal to make OP_RUN a part of the DL standard again: driplineorg/dripline#37

Updates needed

• dripline_constants.hh: Add constant to op_t
• dripline_constants.cc: Fix to_string() and to_op_t()
• endpoint.hh (60-ish): Update docs to include OP_RUN
• endpoint.cc (129): Add OP_RUN to switch block
• hub.cc/hh: Add remove_run_handler()
• agent.cc (430): Add agent::sub_agent_run::create_request()
• dl-agent.cc (52): Add subcommand for OP_RUN