Existing scripts and associated config such as rc.initdiskless, rc.d/var, and others, use mdmfs to create memory filesystems. That program accepts a size argument which allows SI suffixes and treats an unsuffixed number as a count of 512 byte sectors. That makes it difficult to convert existing scripts to use tmpfs instead of mdmfs, because tmpfs treats unsuffixed numbers as a count of bytes. The script logic to deal with existing user config that might include suffixed and unsuffixed numbers is... unpleasant.
Also, there is no g'tee that tmpfs will be available. It is sometimes configured out of small-resource embedded systems to save memory and flash storage space.
These changes enhance mdmfs(8) so that it accepts two new values for the 'md-device' arg: 'tmpfs' and 'auto'. With tmpfs, the program always uses tmpfs(5) (and fails if it's not available). With 'auto' the program prefers tmpfs, but falls back to using md(4) if tmpfs isn't available. It also handles the -s <size> argument so that the mdconfig interpetation of unsuffixed numbers applies when tmpfs is used as well, so that existing user config keeps working after a switch to tmpfs.
A new rc setting, mfs_type, is added to etc/defaults/rc.conf to let users force the use of tmpfs or md; the default value is "auto".