Make `shellInit` option and friends follow documentation

# Synopsis

First, let me apologize for dropping this non-trivial, unsolicited pull request. This pull request tries to address the following problems:
1. `programs.bash.shellInit` is actually only read for login bash shells.
2. There are settings in `environment.shellInit` that should only be read for login shells. On bash, this is okay (see above), but on shells that actually support the `shellInit` option, you get a pristine login shell with every invocation, whether you want it or not.
3. The `environment.shellInit` option is not actually shell-agnostic for the above reasons.

# Proposed solution

As a solution, I propose the following four patches:

(935c181) Remove `programs.bash.shellInit` option:

> The `shellInit` is supposed to contain settings used to initialize all
> bash shells, but is actually only used for login shells. This is because
> bash does not have a global initialization file for all shells: for
> login shells `/etc/profile` is read and for interactive shells
> `/etc/bashrc` is read, but there is no global initialization file for
> non-interactive, non-login shells. The `shellInit` option should be
> removed for bash because it cannot do what it says.

(12d4fc8) Move login shell settings to `environment.loginShellInit`

> The `shellInit` option was being used for settings that are only
> appropriate for login shells.

(a96ee92) Set zsh environment for login shells only

> Environment variables should be set in `/etc/zprofile` where they will
> only be loaded for login shells. They were being set in `/etc/zshenv`;
> as a result, *every* new zsh invocation was like a new login shell.
> After this change, environment variables are set in zsh as they are in
> bash (as they should be).

(8d07669) Remove `environment.shellInit` option

> The `environment.shellInit` option is supposed to contain initialization
> settings for all invocations of all shells. However, not all shells
> support global initialization settings for all types of invocation.
> Notably, bash does not support this option. Shell-agnostic settings
> should go in `environment.loginShellInit` or
> `environment.interactiveShellInit` instead.

## Pros
* All options now match their documentation.
* Options that cannot actually be implemented are removed.
* bash users (which I think are most NixOS users) without custom settings for `programs.bash.shellInit` or `environment.shellInit` see no change.

## Cons
* Anyone who uses `programs.bash.shellInit` or `environment.shellInit` will have to move their settings to `loginShellInit`. I'm not entirely sure this is a "con", because these users might not be aware of what their settings actually do!

Thanks for reading!