getopts appears to not be shifting $@ when consuming options

[earnestly <zibeon@googlemail.com>]

In this example dash will repeatedly append 'attr=foo' to the list of
parameters in an infinite loop:

    #!/bin/dash -x

    while getopts :a: arg -a foo -a bar; do
        case $arg in
            a) set -- "$@" attr="$OPTARG"
        esac
    done
    shift "$((OPTIND - 1))"

Instead I expected this to result in parameter list containing
'attr=foo' and 'attr=bar'.

This works in all shells I have been able to test with the exception of
busybox sh:

    * sh   (bash)
    * bash (All versions from 1.14 through 5.1.4)
    * mksh (MIRBSD KSH R59 2020/05/16)
    * ksh  (93u+)
    * zsh  (5.8)
    * zsh --emulate sh
    * heirloom-sh (bourne)

The only workaround I've found is to explicitly use `shift 2` in the a)
case and obviate the final shift using OPTIND.  This will unfortunately
break every other shell.