Slight cleanup of interface event logging.

Make if_printf() use vlog() instead of vprintf(). This means it can no
longer return the number of characters printed, as it used to, but every
single call to if_printf() in the entire kernel ignores the return value
anyway; just return 0 so we don't have to change the prototype.

Consistently use if_printf() throughout sys/net/if.c, instead of a
mixture of if_printf() and log().

In ifa_maintain_loopback_route(), don't needlessly log an error if we
either failed to add a route because it already existed or failed to
remove one because it did not. We still return an error code, though.

MFC after: 1 week