This filters out duplicate addresses from address command output.
It also also adds tests for the address command.
The code here is an extended version of a patch from Jani Nikula.
This moves address-related functionality from search command to the
new address command. The implementation shares almost all code and
some command line options.
Options --offset and --limit were intentionally not included in the
address command, because they refer to messages numbers, which users
do not see in the output. This could confuse users because, for
example, they could see more addresses in the output that what was
specified with --limit. This functionality can be correctly
reimplemented for address subcommand later.
Also useless values of --exclude flag were not included in the address
command.
This was inspired by a patch from Jani Nikula.
