abrt-cli: make status help message more precise

From 'abrt-cli list --full' to either 'abrt-cli list --since XXXXX' or
'abrt-cli list' if 'abrt-cli status' was called with --since or not.

Related to #659

Signed-off-by: Jakub Filak <jfilak@redhat.com>
Signed-off-by: Jiri Moskovcak <jmoskovc@redhat.com>