ash: make ${#var} unicode-aware

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
addgroup: mention -S in the help text

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
taskset: support CPU masks for more than 64 CPUs

function                                             old     new   delta
taskset_main                                         522     631    +109

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