Choice::refused was cleared only when the agent accepted a
session/set_config_option, so it outlived what it described in two cases:
a respawned agent process kept the dead one's refusal masking the wanted
value in the picker, and a refusal survived a turn whose wanted value was
the one the session already held (or none), so re-picking the refused
value snapped the picker back to the session's value.
start() now resets the refusals along with the offered options, and
choose() clears the category's refusal when the wanted value is none or
the one the session holds.
Refs #4832
7ccbeaa161