Pad a version string with zeros to match a given number of components.

It's redundant with `argsOverride.modDirVersion`.

It's not used, doesn't build, and seems like the only reason to have
`manualConfig` take `stdenv` as an argument.

Reverts NixOS@7c7c83e which was
only needed for the minimal-kernel.nix test module and clutters the call site.

stdenv can still be overridden with `linuxManualConfig.override { stdenv = ...; }`.

Document the `linux.override` way first, then `linuxManualConfig`.

Add a `linux.configEnv` passthru attribute for quickly getting a
`make nconfig`-ready shell.