Advertising 127.0.0.1 host candidates to remote peers is useless and
harmful: unreachable candidates leak host internals in SDP and make
remote agents waste TURN permissions and connectivity checks on dead
candidates (a same-host TURN relay can even forward them back to
itself, pinning calls onto a broken path).

Default gathering now skips loopback for UDP hosts, TCP active
placeholders, and TCP passive listeners. Set
ice_include_loopback_candidates = true to restore the old behavior;
an explicit bind_ip = "127.0.0.1" is unaffected.

Same-host tests that relied on loopback candidates (external_ip
rewriting tests, local TURN relay test) opt back in explicitly.