mdmfs(8): add -k skel option to populate fs from a skeleton

mdmfs(8) lacks the ability to populate throwaway memory filesystems from an
existing directory.

This features permits an interesting setup where /var for instance lives on
a device where wear-leveling is something you want to avoid as much as
possible and nonetheless you don't want to lose your logs, ports metadata,
etc. Here are the steps:

1. Copy /var to /var.bak;
2. Mount an mfs into /var using -k /var.bak at startup;
3. Synchronize /var to /var.bak weekly and on shutdown.

Note that this more or less mimics OpenBSD's mount_mfs(8) -P flag.

PR: 146254
Submitted by: jlh (many moons ago)
MFC after: 1 week