[PATCH] Prefer HTTPS to HTTP and FTP in texinfo.tex

[Paul Eggert]

In Gnulib, Emacs, etc. we are changing ftp: and http: URLs to use https:, to 
discourage man-in-the-middle attacks when downloading software. The attached 
patch propagates these changes upstream to texinfo.tex. This patch doesn't 
affect the the other files in the texinfo source, though they should be updated 
too at some point.

Although texinfo.tex is not secret, plain HTTP is vulnerable to malicious 
routers that tamper with responses from GNU servers, and this sort of thing is 
all too common when people in some other countries browse US-based websites. 
See, for example:

Aceto G, Botta A, Pescapé A, Awan MF, Ahmad T, Qaisar S. Analyzing internet 
censorship in Pakistan. RTSI 2016. https://dx.doi.org/10.1109/RTSI.2016.7740626

HTTPS is not a complete solution here, but it can be a significant help. The GNU 
project regularly serves up code to users, so we should take some care here.

texinfo.tex.diff
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