Update for NetBSD 6.1.

Also use $((foo)) syntax rather than expr(1) for calculations.
Use ftp.jp.NetBSD.org.
Bump revision and copyright year.
Also create a bootable floppy image with the secondary netboot.
Add .gitignore file that contains working directories etc.
Update descriptions for 6.1, i.e. note about a bootable netboot floppy image.