kern/51357: cu(1) fails to connect, kills networking and is unkillable

[roy%marples.name@localhost]

>Number:         51357
>Category:       kern
>Synopsis:       cu(1) fails to connect, kills networking and is unkillable
>Confidential:   no
>Severity:       serious
>Priority:       high
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Mon Jul 25 11:55:00 +0000 2016
>Originator:     Roy Marples
>Release:        NetBSD-7.99.34
>Organization:
>Environment:
NetBSD uberlaptop2.marples.name 7.99.34 NetBSD 7.99.34 (GENERIC) #2: Mon Jul 25 12:15:24 BST 2016  roy%uberlaptop2.marples.name@localhost:/home/roy/obj/src-amd64/sys/arch/amd64/compile/GENERIC amd64
>Description:
$ cu -s 115200 -l ttyU2
Connected

On NetBSD-7 the above command works, on -current Connected is not shown, networking fails (can no longer ping 127.0.0.1 for example) and the cu process is not killable even with kill -9.
>How-To-Repeat:

>Fix: