bug#64423: 29.0.92; save-interprogram-paste-before-kill doesn't prevent streaming large selections

[sbaugh]

1. emacs -Q under X, preferably X forwarded over ssh to make things slower.
2. (setq save-interprogram-paste-before-kill 2000) (or any other integer)
3. Copy some very large data in another X client, so the selection is
very large.
4. (kill-new "foo")
5. Observe Emacs hanging as it receives the entire large data from the
selection owner, and then after receiving it all, discards it because
it's more than 2000 bytes.

Solution: receive_incremental_selection in xselect.c should support a
cap on the size of the selection it receives and truncate (or discard,
returning nil?) the selection if it's larger than that.  And setting
save-interprogram-paste-before-kill to a numeric value should activate
this cap.



In GNU Emacs 29.0.92 (build 2, x86_64-pc-linux-gnu, X toolkit, cairo
 version 1.16.0, Xaw3d scroll bars) of 2023-07-01 built on earth
Repository revision: b179926388ee76f7b3304535a7189f89bd7c7f8c
Repository branch: emacs-29
Windowing system distributor 'The X.Org Foundation', version 11.0.12014000
System Description: NixOS 22.11 (Raccoon)

Configured using:
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