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.
Merge branch 'master' into KDP
KDP: Ensure that all other BSEL2 bits are clear when setting RDI.

Found in DMR by Mark P, confirmed in KDP ucode.

Note that, at least for DMR, the VMS driver does a BISB to set the command code, which relies on this.