pwclient: require one or more of pwclient -{a,s}

pwclient: require one or more of pwclient -{a,s}

Previously, we required the -s flag for 'pwclient update'. But since we
allow updating up to 2 different fields ('archived' and 'state'), drop
the required flag, and just enforce that the user must provide -a, -s,
or both.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Stephen Finucane <stephen.finucane@intel.com>