This includes:
* making bridge_info_t exposed but opaque
* allowing guards where we don't know an identity
* making it possible to learn the identity of a guard
* creating a guard that lacks a node_t
* remembering a guard's address and port.
* Looking up a guard by address and port.
* Only enforcing the rule that we need a live consensus to update
the "listed" status for guards when we are not using bridges.
This patch is just:
* Code movement
* Adding headers here and there as needed
* Adding a bridges_free_all() with a call to it.
It breaks compilation, since the bridge code needed to make exactly
2 calls into entrynodes.c internals. I'll fix those in the next
commit.
