ash: [BUILTIN] Treat OPTIND=0 in the same way as OPTIND=1

ash: [BUILTIN] Treat OPTIND=0 in the same way as OPTIND=1

Upstream commit:

    Date: Sat, 6 Oct 2007 18:59:31 +0800
    [BUILTIN] Treat OPTIND=0 in the same way as OPTIND=1

    Previously setting OPTIND to 0 would cause subsequent getopts calls to fail.
    This patch makes dash reset the getopts parameters the same way as OPTIND=1.

    Both behaviours are allowed by POSIX but other common shells do tolerate this
    case.

function                                             old     new   delta
getoptsreset                                          24      30      +6
getoptscmd                                           632     614     -18

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>