a_tty_kht(): take advantage of new n_SHEXP_STATE_WS_TRAIL

a_tty_kht(): take advantage of new n_SHEXP_STATE_WS_TRAIL

until now "? File <TAB>" would have tried to expand "File " because, hm,
of the pre IFS era failure to treat WS as IFS, thus the last real token
we have seen is "File ", but this is not what the user wants to expand!
It would have worked if a any quote would have been used, e.g. "File \"<TAB>",
but this is also not what a user wants.

We can now take advantage of the new n_SHEXP_STATE_WS_*, and if the last
token parse has not seen trailing whitespace then we can indeed
look at the last character, and if that is a whitespace (TODO not
mbyte/Unicode yet) then we can dare to use that as a desired
delimiter.  In effect "File <TAB>" will thus expand "*" instead.

While here, do not start any token parse if all we have seen is WS.