hush: make ${#var} unicode-aware

This mimics bash

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
ash,hush: run reinit_unicode() only if makes sense

With static Unicode support, no need to check $LANG et al.

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