Don't update mtab upon mounting procfs
Don't update mtab upon mounting procfs

It causes an ugly error message when booting from cd and it gets
overwritten by `printroot >/etc/mtab` in /etc/rc anyway.

veritas-shine/minix3-rpi,veritas-shine/minix3-rpi,veritas-shine/minix3-rpi,veritas-shine/minix3-rpi,veritas-shine/minix3-rpi,veritas-shine/minix3-rpi,veritas-shine/minix3-rpi,veritas-shine/minix3-rpi