Draft: SCTP problems: allow to bind separate F1-C/F1-U interfaces; allow to use DNS

• revert commit to fix issue #727 (closed)
• introduce new option to bind on a separate address for F1U; if not specified, will reuse F1C; fix in CI; add doc
• rewrite SCTP functions to use C's getaddrinfo() to look up DNS names. This also allows to IPv6, but more work in the stack is necessary to fully use IPv6 (C-plane works, U-plane doesn't)
• remove all interface name config options (are not used), remove ipv6 config options (handled through ipv4)

Closes #727 (closed)