Allow disabling resume history display and clear old history on re-resume

[nhicks00]

Currently the resume history display (shown on /resume, /autosave_load, /load_context) has two limitations:

1. The minimum resume_message_count is 1 — there's no way to disable it entirely
2. If you resume multiple sessions in a row, old history stays on screen and new history stacks below it

This change:

• Allows resume_message_count=0 to disable the display
• Automatically clears previously displayed history when resuming a new session (using ANSI cursor save/restore)