This only supports two addresses as the intended use is 1 IPv4 and 1 IPv6
address on a single-homed host, and the only supported configuration of
outgoing connections to other servers is to bind a single IPv4 or IPv6
address.
If an IPv4 connection is dropped by the pre-callback, and there is a
pending IPv6 connection on the same listening socket then the retried
accept() will be unable to populate `st` because `addrlen` will be too
small. Also initialise `st` each time to avoid a clang static analysis
warning.
These include warnings about "break" statements that will never be
executed (because they are after "return" statements), unused macros
(lost to code refactoring or never even used in the first place),
functions that call abort() or loop indefinitely but aren't marked with
the "noreturn" attribute, and use of variables possibly uninitialised
(a false positive).
