client: Enable history file by default unless -r

client: Enable history file by default unless -r

Make -H take an argument.  Default it to ~/.empire_history, except in
-r restricted mode, where history is off unless you specify -H.
That's because restricted mode restricts the player's access to the
local system, and that includes the history file.  If you want to
grant access to a history file, you have to do so explicitly.

Thanks to the previous commit, there is no need to suppress saving to
~/.empire_history in the test suite.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>