Rename the 40col menus from .asc to .msg so they appear for CBM/PETSCII terms

The menu file selection priority is like this (for 40col CBM/PETSCII terminals):
main.40col.seq
main.seq
main.40col.msg
main.msg
main.40col.asc
main.asc

so the main.msg was being chosen before main.40col.asc.

Plus, these files contain ex-ASCII chars, so .msg is actually the more correct
file extension anyway.