bash: Fix bashisms being applied to non-bash shells

[TraceyC77]

Summary

Move sourcing of bashrc.d directory to default skel/.bashrc
Make all scripts in profile.d POSIX compliant
Fixes issues with errors being emitted by non-bash shells
Fixes bash specific user scripts improperly being loaded by other shells
Add README for profile.d files needing to be POSIX compliant

Test Plan

Installed the new eopkgs & rebooted. Verified:
bash loads properly with no errors
Scripts in ~/.bashrc.d/ loaded
zsh loads properly with no errors
Scripts in
Scripts in /usr/share/defaults/etc/profile were loaded for both shells

Checklist

• Package was built and tested against unstable

Fixes #4214

[TraceyC77]

There is one problem I still need to solve. The block to source ~/.bashrc.d has been moved from /usr/share/defaults/etc/profile to /etc/skel/.bashrc so that a user's bash scripts are only sourced by bash, not all shells. This will be find for new users on a system, but we need a way to add that block for existing users on the system. I am not sure how to approach that, ideas are welcome

[ermo]

As requested:

Gentle suggestion to perhaps also consider adding zsh /etc/skel files that support ~/.zshrc.d/* auto-loading?

Another (alternative?) option is to check the shell being run in the vendor scripts under /usr/share/defaults, and then, depending on shell, auto-loading ~/{ba,z}shrc.d/* files?

[TraceyC77]

Gentle suggestion to perhaps also consider adding zsh /etc/skel files that support ~/.zshrc.d/* auto-loading?

I can add that to the .zshrc file, similar to how .bashrc loads ~/.bashrc.d/*
This is pretty standard

Another (alternative?) option is to check the shell being run in the vendor scripts under /usr/share/defaults, and then, depending on shell, auto-loading ~/{ba,z}shrc.d/* files?

That is not standard, and fragile. If you load a shell, it loads its startup files (like .bashrc / .zshrc) which then (per this PR) load the scripts directory. What you suggest would add unnecessary complication IMO.

Thanks for the feedback :)