added man page for diskfile

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+overlay-diskfile(8)
+===================
+:doctype: manpage
+:revdate: {sys:date "+%Y-%m-%d %H:%M:%S"}
+:COLON: :
+:EQUALS: =
+
+NAME
+----
+overlay-diskfile - Support for using diskfile filesystem.
+
+SYNOPSIS
+--------
+LOWER=!*overlay-diskfile* _mount/path_ [ _diskfile_ [ _index_ ] ]
+UPPER=!*overlay-diskfile* _mount/path_ [ _diskfile_ [ _index_ ] ]
+
+DESCRIPTION
+-----------
+
+*overlay-diskfile* is a support script for pre-mounting a diskfile to
+provide the LOWER or UPPER filesystem for a subhost. It is typically
+used in the subhost configuration to be invoked as part of determining
+the filesystem path.
+
+OPTIONS
+-------
+
+no options.
+
+EXAMPLES
+--------
+
+.Use a whole diskfile filesystem as LOWER
+====
+----
+LOWER=!overlay-diskfile mnt/ disk.img
+----
+====
+
+The above mounts the disk.img filesystem onto +mnt+ and provides that
+as the LOWER filesystem.
+
+.Use a diskfile partition 2 as LOWER
+====
+----
+LOWER=!overlay-diskfile mnt/ disk.img 2
+----
+====
+
+The above mounts the second partition onto +mnt+ and provides that as
+the LOWER filesystem.
+
+.Use a subtree of a diskfile partition 2 as LOWER
+====
+----
+LOWER=!overlay-diskfile mnt/some/path disk.img 2
+----
+====
+
+The above mounts the second partition onto +mnt+ and provides the path
++mnt/some/path+ that as the LOWER filesystem.
+
+.Use a while diskfile as UPPER and WORK.
+====
+----
+UPPER=!overlay-diskfile mnt/some/upper/path disk.img 2
+WORK= mnt/some/work/path
+----
+====
+
+The above mounts the second partition of disk.img and uses
++mnt/some/upper/path+ as UPPER and +mnt/some/work/path+ as WORK.
+
+
+SEE ALSO
+--------
+
+*overlay-boot*, *overlay-stop*