Fixes on circuitbuild.[ch] based on nick's comments.

Fixes on circuitbuild.[ch] based on nick's comments.

* Renamed transport_info_t to transport_t.
* Introduced transport_get_by_name().
* Killed match_bridges_with_transports().
  We currently *don't* detect whether any bridges miss their transports,
  of if any transports miss their bridges.
* Various code and aesthetic tweaks and English language changes.