growfs_fstab startup script: set dumpdev to AUTO

The growfs_fstab script has been testing dumpdev, and if it is AUTO,
enables dumps on the newly-added swap device for the initial boot.
However, dumpdev defaults to AUTO on main, but NO on stable/13 and
release branches. On the other hand, bsdinstall adds dumpdev="AUTO"
by default (controlled by a menu item). bsdinstall is not used when
booting an SD card or other disk image. Adopt the default from
bsdinstall, and set dumpdev to AUTO in /etc/rc.conf in the
growfs_fstab script if a swap partition has been added, along with
the explanatory comment added by bsdinstall.

Differential Revision: https://reviews.freebsd.org/D38751