newroot_premount[_essential](): prefix want_mp

newroot_premount_essential() is an irun-wrapped variant of newroot_premount().
lin/initramfs/mount: imount_fs(): specify argv

This is a documentation change only.
liram/base: liram_populate_inherit()

liram_populate_inherit()

Also added liram_populate_helper(), which calls a layout-specific helper
function.

(TODO) See the docs or liram/layout/README for details.
liram/layout: add README file

Some documentation about liram layouts.

(TODO) This should be merged with /doc/shlib.rst.
lib/itertools: file_iterator: check if file exists

Also added the file_list_iterator() function which calls file_iterator()
with some fixed keyword args.
initramfs/newroot: add setup module

This module provides some functions for setting up dirs and mounts in newroot.
IOW, it realizes file-system based configuration of newroot.
liram: add 'subtree' module

Currently, this module is able to mount liram so-called subtrees,
which are basically tmpfs mounts.
liram: add 'setup' module

In addition to what's provided by initramfs/newroot/setup, this module
implements subtree mounting.
liram/core: export functions subtree and setup
liram/layout/default: call setup functions

This is a backwards-compatible change to the default layout which will create
extra directories and mount liram subtrees in newroot if the rootfs demands it.