transport server: bind to the address family enabled on the host

[Narasimha-sc]

Connecting a mobile to the desktop fails when IPv6 is disabled in the kernel (ipv6.disable=1) — simplex-chat/simplex-chat#5515, simplex-chat/simplex-chat#3531.

startTCPServer resolves the wildcard address with AI_PASSIVE only, and then select picks the IPv6 address whenever one is returned:

let hints = defaultHints {addrFlags = [AI_PASSIVE], addrSocketType = Stream}
 in select <$> getAddrInfo (Just hints) host (Just port)
select as = fromJust $ family AF_INET6 <|> family AF_INET

getaddrinfo returns :: even when the host has no IPv6 at all, so the filter selects it and socket AF_INET6 throws EAFNOSUPPORT.

AI_ADDRCONFIG is the missing part of the filter: it excludes the address families that have no address configured on any interface. This adds startTCPServerConfigured, which passes it, and uses it for the remote control TLS server — the only listener that has to accept connections on an IPv6-less host, as its address is advertised as IPv4 in the invitation.

startTCPServer itself is unchanged: SMP/XFTP/ntf servers and the local TCP server keep requiring IPv6, and still fail if it is disabled.

Measured

Calling the two functions from this branch with host = Nothing:

host state	startTCPServer	startTCPServerConfigured
IPv6 enabled	[::]	[::]
ipv6.disable=1 (no IPv6 addresses, socket(AF_INET6) → EAFNOSUPPORT)	Network.Socket.socket: unsupported operation (Address family not supported by protocol)	0.0.0.0
net.ipv6.conf.all.disable_ipv6=1	[::]	0.0.0.0

Verified in a network namespace — the third row with the real sysctl, the second with an LD_PRELOAD shim making socket(AF_INET6) return EAFNOSUPPORT on top of a namespace with no IPv6 addresses. Also checked that AI_ADDRCONFIG never empties the result: on a host with no addresses configured at all glibc falls back to returning both families, so fromJust in select stays safe.

Supersedes the earlier catchAny-on-bind version of this PR, which worked around the selection instead of fixing it.