Add router_have_consensus_path() as status/consensus-exit GETINFO control event

Split from legacy/trac#13718 (moved):

We could add router_have_consensus_path() as a GETINFO control event called status/consensus-exit or something. But this is low priority, as knowing that the consensus is missing exits only impacts on test networks.

These details have been obsoleted by changes in legacy/trac#13718 (moved): > We may also need to update the status/enough-dir-info GETINFO control event - should we add status/enough-dir-info/exit and status/enough-dir-info/internal (we default status/enough-dir-info to exit for backwards compatibility).

nickm: Sounds fine, though it could be a separate ticket.

Note: the relevant code is located in src/or/control.c in getinfo_helper_events() around line 2019.