Improve missing tty handling in init(8). This removes a check that did
nothing - it was checking for ENXIO, which, with devfs, is no longer
returned - and was badly placed anyway, and replaces it with similar
one that works, and is done just before starting getty, instead of being
done when rereading ttys(5).

From the practical point of view, this makes init(8) handle disappearing
terminals (eg /dev/ttyU*) gracefully, without unneccessary getty restarts
and resulting error messages.

Reviewed by: imp@
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D14307