Don't allow opening the callout device when the callin device is already
open (in disguise as the console device). The only allowed combination
was supposed to be the callin device with the console.

Fix the assertion in ttydev_close() that was meant to detect this (it
only detected all 3 devices being open). Assert this in ttydev_open()
too.

Submitted by: bde
MFC after: 2 weeks