stdenv-setup: Clean up 'substitute()' for style and error handling

substitute() {

    local input="$1"

    local output="$2"

    local input=$1

    local output=$2

    shift 2

    if [ ! -f "$input" ]; then

      echo "substitute(): file '$input' does not exist"

      echo "${FUNCNAME[0]}(): ERROR: file '$input' does not exist" >&2

      return 1

    fi

    local -a params=("$@")

    local n p pattern replacement varName content

    # a slightly hacky way to keep newline at the end

    content="$(cat "$input"; printf "%s" X)"

    content="${content%X}"

    for ((n = 2; n < ${#params[*]}; n += 1)); do

        p="${params[$n]}"

        if [ "$p" = --replace ]; then

            pattern="${params[$n + 1]}"

            replacement="${params[$n + 2]}"

            let n+=2

    local content

    # read returns non-0 on EOF, so we want read to fail

    if IFS='' read -r -N 0 content < "$input"; then

        echo "${FUNCNAME[0]}(): ERROR: File \"$input\" has null bytes, won't process" >&2

        return 1

    fi

        if [ "$p" = --subst-var ]; then

            varName="${params[$n + 1]}"

            let n+=1

    while (( "$#" )); do

        case "$1" in

            --replace)

                pattern=$2

                replacement=$3

                shift 3

                ;;

            --subst-var)

                local varName=$2

                shift 2

                # check if the used nix attribute name is a valid bash name

                if ! [[ "$varName" =~ ^[a-zA-Z_][a-zA-Z0-9_]*$ ]]; then

                echo "WARNING: substitution variables should be valid bash names,"

                echo "  \"$varName\" isn't and therefore was skipped; it might be caused"

                echo "  by multi-line phases in variables - see #14907 for details."

                    echo "${FUNCNAME[0]}(): WARNING: substitution variables should be valid bash names," >&2

                    echo "  \"$varName\" isn't and therefore was skipped; it might be caused" >&2

                    echo "  by multi-line phases in variables - see #14907 for details." >&2

                    continue

                fi

            pattern="@$varName@"

            replacement="${!varName}"

        fi

                pattern=@$varName@

                replacement=${!varName}

                ;;

        if [ "$p" = --subst-var-by ]; then

            pattern="@${params[$n + 1]}@"

            replacement="${params[$n + 2]}"

            let n+=2

        fi

            --subst-var-by)

                pattern=@$2@

                replacement=$3

                shift 3

                ;;

            *)

                echo "${FUNCNAME[0]}(): ERROR: Invalid command line argument: $1" >&2

                return 1

                ;;

        esac

        content="${content//"$pattern"/$replacement}"

    done