server: Deprecate setatt in favor of touch.

server: Deprecate setatt in favor of touch.

This adds --set-attribute and --unset-attribute to the touch subcommand,
re-implementing the features of the setatt command with a saner syntax.

We augment the change_atts_data structure using pre-computed values for
verbose and dry-run mode and pass a pointer to this structure rather than
the general callback arg pointer. The existing setatt subcommand neither
sets nor consults the two new booleans.

The touch completer of para_client is updated to complete the two new options,
executing the lsatt subcommand to get the attribute names. The manual and
the test suite also need minor adjustments.