add an option to not strip filesystem prefix from file names

[Jean-Baptiste Lallement]

The filesystem prefix of the files in the main/ directory is not preserved when an initrd is unpacked with unmkinitramfs.

If the initrd is rebuilt directly from main/ with cpio, the resulting file won't work because links are invalid.

unmkinitramfs should have an option to not strip filesystem prefixes and preserve the content of the archive..

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: initramfs-tools-core 0.133ubuntu11
ProcVersionSignature: Ubuntu 5.3.0-19.20-generic 5.3.1
Uname: Linux 5.3.0-19-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.11-0ubuntu9
Architecture: amd64
CurrentDesktop: ubuntu:GNOME
Date: Wed Nov 27 09:40:24 2019
InstallationDate: Installed on 2014-07-15 (1961 days ago)
InstallationMedia: Ubuntu 14.10 "Utopic Unicorn" - Alpha amd64 (20140520)
PackageArchitecture: all
SourcePackage: initramfs-tools
UpgradeStatus: Upgraded to focal on 2018-03-24 (612 days ago)