Additional tweaks for Lmod-based init scripts

[ocaisa]

There are a few things that can be done to further improve the Lmod-based init scripts.

• Make the bash, zsh and ksh initialisation scripts POSIX compliant
  • This would also allow us to have a unique script with symlinks for the others (including a new sh variant which should also work with dash). The only thing that is unique to each script is the name of the shell, since the script is (typically) sourced $0 is the name of the current shell (with potentially a leading -). We need the shell to know what initialisation script to use from Lmod. It should be possible to store the shell as $(basename -- ${0#-}) which will work for both source and direct execution since the filename and shell are meant to be the same (and even better, for a sourced script it will be forgiving even if you call the "wrong" script). dash would need to be special-cased (dash -> sh).
• Currently, if you follow the sequence

  ocaisa@~$ source /cvmfs/software.eessi.io/versions/2025.06/init/lmod/bash
  Modules purged before initialising EESSI
  Module for EESSI/2025.06 loaded successfully
  EESSI has selected aarch64/neoverse_n1 as the compatible CPU target for EESSI/2025.06
  EESSI did not identify an accelerator on the system
  (for debug information when loading the EESSI module, set the environment variable EESSI_MODULE_DEBUG_INIT)
  {EESSI/2025.06} ocaisa@~$ source /cvmfs/software.eessi.io/versions/2023.06/init/lmod/bash
  Running "module reset". Resetting modules to system default. The following $MODULEPATH directories have been removed: None
  Module for EESSI/2025.06 loaded successfully
  {EESSI/2025.06} ocaisa@~$
  

  you end up with the "wrong" EESSI version loaded as EESSI_VERSION was set when you called the second script (which is an override variable). This needs to be special cased.

[ocaisa]

My AI friend says it is better to do

if [ -n "${BASH_VERSION-}" ]; then
    shell=bash
elif [ -n "${ZSH_VERSION-}" ]; then
    shell=zsh
elif [ -n "${KSH_VERSION-}" ]; then
    shell=ksh
else
    shell=sh
fi

which covers all the POSIX-compatible cases