kakapo-lw-search: more intuitive variable name
make `kakapo-soft-tab' rely on indent-tabs-mode

Also, rename it to `kakapo-hard-tab'.
kakapo-all-ktab: simplify code

No functional change.
kakapo-lw-search: more aggressive return value

Before, we only used the searched leading whitespace value if the
current line's leading whitespace was empty. Instead, we return
whichever one is greater.