bug, refactor, test: make pragma history_length work interactively history_length could be set interactively, but it was never used to set readline/pyreadline3's internal state. Using the pragma setting on the roundup-admin command line did set readline's state. Also refactored 2 calls to self.readline.get_current_history_length() into one call and storing in a variable. Also changed method for creating history strings for printing. Tests added for history_length pragma on cli and interactive use. Added test for exiting roundup-admin with EOF on input. Added test for 'readline nosuchdirective' error case. Added test to readline with a command directive to set an internal variable. This last one has no real test to see if it was successful because I can't emulate a real keyboard/tty which is needed to test.

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