Cleanup fastbin_dup_consolidate and prepare it for glibc version 2.41

[k4lizen]

Handles: https://github.com/shellphish/how2heap/issues/200

Clarified some language to make it more clear this isn't a technique that (in general) gives tcache double free -> tcache poison. Cleaned up the explanation a bit.

I didn't touch versions 2.24 and 2.23 (which don't have tcache).