comm etc.: Provide comm_addr_equal

Abolish the rule that a comm_addr has zeroes in all its holes.
Provide comm_addr_equal instead.

We can get rid of a lot of calls to FILLZERO.

In resolver.c we no longer need to copy the fields of ia individually.
We still need to look at the incoming address family since util.c
aborts on unknown AFs and adns (perhaps a new version or something)
might have sent us things we don't understand.  (Also reorganise the
loop/switch a little to get `wslot++' out of the `case'.)

We have to move the declaration of iaddr_equal.

Abolish transport_addrs_equal and replace it at call sites with the
new comm_addr_equal.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>