The problem with vlock is that it pulls in PAM, and PAM is quite large,
while kbd is a pretty basic package that might be very useful for basic
console-related activities, such as setting font during early boot.

util-linux minial is meant to be minimal and PAM is not minimal.

This is mostly a readability commit.

It essentially doesn’t change anything in the expression, except that
it splits the expression parameters into logical groups and reorders
the configuration options to split them into logical groups as well
and to have them in the order that more closely resembles that of
the upstream documentation and configuration scripts.

The only two changes to the semantics of the expression are:

* `withSelinux` is removed and replaced with `selinux != null`.
* `lz4` is not forced-enabled, instead the build system will detect
  whether it is available or not.

This allowd for a minimalistic build of systemd for embedded
applications or stage1 initramfs.

Make sure the following optional dependencies are truly optional:

* pam
* gnu-efi
* libidn2
* curl
* gnutar
* gnupg

Add a missing importd dependency (also optional):

* zlib

* The debug shell is bash by default, but make it configurable, so that
  ash from busybox can be used instead, for example.

* It is very fat and not needed for a minimalistic build.