vt(4): When cutting a line, append a newline character.

While at it optimise "case 3" into a default.
This way there is no need to initialize the "mark" variable in the beginning,
because all cases set it.

Sponsored by: NVIDIA Networking
Differential Revision: https://reviews.freebsd.org/D36042

(cherry picked from commit 90b89100548f7fef4cbd4a8c76dc3d83ab786d7a)