Add the ability for full installs to optionally use RAM disks for /tmp and /var, and to allow selecting a size for /tmp and /var RAM disks on both Full installs and NanoBSD.
I think I caught most of the edge cases for transitioning into and out of RAM disk mode, and preserving data across reboots as is done on NanoBSD (RRD, DHCP leases, pkg/pbi info).