indexing: Change from try_decrypt to decrypt

the command-line interface for indexing (reindex, new, insert) used
--try-decrypt; and the configuration records used index.try_decrypt.
But by comparison with "show" and "reply", there doesn't seem to be
any reason for the "try" prefix.

This changeset adjusts the command-line interface and the
configuration interface.

For the moment, i've left indexopts_{set,get}_try_decrypt alone.  The
subsequent changeset will address those.