growfs_fstab: add new /etc/rc.d script to add swap to fstab

The growfs_fstab script is a helper for the growfs script to add any
new swap partition to /etc/fstab on first boot. If growfs adds a
swap partition, it sets growfs_swap_pdev in the kenv. In this case,
after the root file system is read/write, if there is no swap partition
in the fstab, growfs_fstab adds growfs_swap as a swap partition to the
fstab. Also, it runs dumpon to add the swap partition (as this
happened earlier in the startup sequence).

Discussed with: cperciva
Differential Revision: https://reviews.freebsd.org/D37463