swim: wrap sio_strfaddr()

SIO provides a function sio_strfaddr() to obtain string
representation of an arbitrary struct sockaddr. Call of this
function usually looks bulky because it requires explicit cast to
const struct sockaddr *, and expects the address size in the
second paremeter.

SWIM uses only AF_INET addresses and always casts them to
const struct sockaddr * + passes sizeof(struct sockaddr_in) to
each invocation of sio_strfaddr(). This patch wraps
sio_strfaddr() with a function making these preparations.

Part of #3234