XIA: add important comment to struct xia_sock

The reason why there's no field xia_dsink in struct xia_sock is not
trivial, and should be justified to avoid one to change the code
to only later realize the why.
XIA/Serval: add socket state to route netlink dump

The socket state is a helpful information for diagnosis and
debugging.

Serval's header (i.e. <net/xia_serval.h>) is adjusted to be
included in userland by applications.
XIA/Serval: fix Serval's close()

Serval's close() didn't work when sk->sk_lingertime == 0 and
related conditions.

This patch also includes a review of serval_sal_close(), and drops
some dead code enclosured by define ENABLE_TCP_REPAIR.