feat(server): give irtt-server sensible zero-argument defaults

With no --bind, the server now binds the wildcard IRTT port on both
address families ([::]:2112 then 0.0.0.0:2112) instead of requiring an
explicit address, on platforms where wildcard reply-source selection
is already supported. Any explicit --bind fully replaces the default
pair rather than adding to it; repeated explicit binds are unaffected.

If one of the two default listeners fails because its address family
has no local support on this host at all (IPv6 disabled on an
otherwise IPv4-capable host, for example), the server falls back to
serving just the surviving family instead of failing outright. Any
other failure reason -- the port already in use, permission denied, no
safe wildcard reply-source path -- is not a family-support problem and
is not silently worked around; startup fails as it would for an
explicit --bind. This distinction is made with the new
irtt_server::address_family_available, which actually binds the
wildcard address of the failed listener's family on an ephemeral port:
a bare socket can construct successfully even when every bind for that
family fails (Linux administratively disabling IPv6 per-interface
leaves AF_INET6 sockets constructible but every bind EADDRNOTAVAIL), so
only a real bind attempt is a reliable signal.

crates/irtt-server/src/lib.rs

@@ -32,6 +32,9 @@
 //! [`ServerConfig`] — `max_sessions` included — applies **per listener**, not
 //! per process.
 //!
+//! See `examples/` in the repository for a runnable single-listener and
+//! multi-listener example.
+//!
 //! # Rejection is silence
 //!
 //! The protocol defines no error reply, no reset and no NACK. Malformed,
@@ -129,4 +132,4 @@ pub use config::{
 pub use core::{OutboundDatagram, ServerCore};
 pub use error::ServerError;
 pub use runtime::{Server, ServerRuntimeError};
-pub use set::{ServerSet, ServerSetError};
+pub use set::{address_family_available, ServerSet, ServerSetError};