/etc/shadow: fix file permissions

[MilhouseVH]

1. Now that /etc/shadow is a symbolic link to /storage/.config/shadow the correct file to chmod 000 in the image is /usr/config/shadow. This also avoids the following warning when running fakeroot:

chmod: cannot operate on dangling symlink '/home/neil/projects/LibreELEC.tv/build.LibreELEC-Generic.x86_64-9.0-devel/image/system/etc/shadow'

2. Ensure that /storage/.config/shadow always has the correct 000 file permission.

[MilhouseVH]

Added a second commit that ensures we only match the user at the beginning of the line. For instance, if we tried to add a user avahi-daemon with the old method it would not be added because:

avahi:x:495:495:avahi-daemon:/var/run/avahi-daemon:/bin/sh

[MilhouseVH]

Third commit: if we add or remove a user from the build, this won't be reflected in the users personal /storage/.config/shadow file which will be missing the new user (in the case of an add), or still referencing the no longer valid user (in the case of a remove, ie. the user is no longer being added during the build).

This third change always overwrites any existing /storage/.config/shadow file with the default values, while retaining any existing root details from the users /storage/.config/shadow file.