LP20 emulation was overwriting existing files

When an LP20 is attached to an existing file, the file is opened in
update mode.  Since the initial file position is zero, any existing
output is over-written.  This is not intuitive.

This edit makes the LP20 seek to EOF on attach.

N.B. Other devices may have cousin issues.
Add set serial to KS cpu

With multiple instances, error logging gets confusing when every CPU has
the same serial number.

This makes the CPU serial number settable.  If not set, the old defaults
apply.
Revert "Add set serial to KS cpu"

This reverts commit 611b7632c993006c5e4bd1d0fe3516cd3f6f4de4.
Add set cpu serial for KS

Allow CPU serial number to be set for KS cpus.

With multiple instances running about, this prevents a lot of confusion,
especially in the error logs.
Change some tabs to spaces
Changed some spaces and indentations
Merge remote-tracking branch 'github/master'