PDP10: Make LP20 support overstrike.

The LP20 emulator never outputs a <CR>.  This makes the ASCII output files unprintable on a TTY-like device.

unix2dos isn't the answer because overprinted lines (text <cr> more text) are lost.  This breaks, among other things, fortran carriage control.

This edit teaches the emulator to output <CR> wherever a TTY-like device would need one to render the file; that is, whenever paper motion occurs outside of column zero.

The resulting output may be: <space><cr><ff> ....<cr><lf><lf><lf>text<cr><lf>
This isn't pure DOS line endings, nor is is pure UNIX.  But it's the minimal format that will drive a TTY, and its the best representation of what the generating system intended.

dos2unix and/or unix2dos can be used to purify the ouput if desired.
PDP10: LP20 should not crash simulator on I/O error

Instead, detach the unit, which has the convenient side-effect of taking it off-line.

This will get the operator's attention in a less painful manner.