IPv6: STCP dual-stack + Android config + invite fixes

- stcp_server: dual-stack (AF_INET6 + IPV6_V6ONLY=0) instead of IPv4-only
- stcp_client: getaddrinfo(AF_UNSPEC) instead of inet_pton(AF_INET)
- stcp_link: detect address family from remote_addr->ss_family
- platform_compat.h: add <netdb.h> for getaddrinfo on POSIX
- ConfigProvider.kt: add [server:main6] addr=[::]:PORT for Android
- invite_link_c.c: invite_serialize_addrs handles IPv6 (was IPv4-only)

lib/platform_compat.h

@@ -166,6 +166,7 @@ extern "C" {
     #include <strings.h>
     #include <arpa/inet.h>
     #include <netinet/in.h>
+    #include <netdb.h>
     #include <fcntl.h>
     #include <unistd.h>
     #include <poll.h>