extindex: do not use current dir like -index does

extindex: do not use current dir like -index does

At least not for resolving inboxes, since there's no good way
for a user to specify what is an inbox or extindex directory
without a command-line switch.

Instead of changing the -extindex command, we change the -index
command internals to rely on the new {-use_cwd} flag to avoid
internal use of negation, since double-negatives and the like
are confusing to me.