don't clobber the user's menu.lst without making a backup first
don't clobber the user's menu.lst without making a backup first

I still don't think our handling of menu.lst makes much sense, but
this is an incremental tweak that fixes one irritation: every time the
default menu.lst changed in the tree, my private version got clobbered.

linusyang/barrelfish,modeswitch/barrelfish,kishoredbn/barrelfish,8l/barrelfish,kishoredbn/barrelfish,BarrelfishOS/barrelfish,BarrelfishOS/barrelfish,mslovy/barrelfish,linusyang/barrelfish,8l/barrelfish,kishoredbn/barrelfish,8l/barrelfish,mslovy/barrelfish,kishoredbn/barrelfish,utsav2601/cmpe295A,mslovy/barrelfish,modeswitch/barrelfish,BarrelfishOS/barrelfish,utsav2601/cmpe295A,mslovy/barrelfish,linusyang/barrelfish,modeswitch/barrelfish,8l/barrelfish,utsav2601/cmpe295A,BarrelfishOS/barrelfish,BarrelfishOS/barrelfish,mslovy/barrelfish,mslovy/barrelfish,utsav2601/cmpe295A,8l/barrelfish,utsav2601/cmpe295A,linusyang/barrelfish,BarrelfishOS/barrelfish,utsav2601/cmpe295A,8l/barrelfish,modeswitch/barrelfish,kishoredbn/barrelfish,modeswitch/barrelfish,BarrelfishOS/barrelfish,linusyang/barrelfish,BarrelfishOS/barrelfish,BarrelfishOS/barrelfish,kishoredbn/barrelfish