Refactor: Improve interface of `entry_is_live`

entry_is_live() checks whether a given entry guard can be used. It accepts as arguments a couple of flags that we might want the entry guard to have.

Its signature is:

static INLINE const node_t *
entry_is_live(const entry_guard_t *e, int need_uptime, int need_capacity,
              int assume_reachable, int need_descriptor, const char **msg)

Which results in calls like this:

      if (entry_is_live(entry, 0, 1, 0, !for_directory, &msg))

     if (entry_is_live(e, 0, 1, 0, 0, &msg))

      const node_t *r = entry_is_live(entry, 0, 1, 0, 0, &live_msg);

It would probably be better if we turn those boolean flags into a bitstring (maybe similar to router_crn_flags_t), so that the calls are more readable.

This is just code refactoring and should result in no behavior changes.