Allow unlimited scrollback/history to extend pass 32bit/2GB limit

Revamp the file-based "unlimited" scrollback code to remove limits
caused by 32bit file offsets. Use Qt's I/O functions which are
buffered and use 64bit offsets. Use Qt's map instead of direct mmap
to ensure consistency. Prevent wrap-around of readWriteBalance.

Patch from FreeBSD committed by rezny

https://github.com/freebsd/freebsd-ports-kde/blob/master/x11/konsole/files/patch-src_History.cpp
https://github.com/freebsd/freebsd-ports-kde/blob/plasma5/x11/konsole/files/patch-fix_infinite_scrollback

CCBUG: 374259