FTP,HTTP → HTTPS in Emacs doc and code

[Paul Eggert <eggert@cs.ucla.edu>]

Inspired by the impending decommissioning of ftp://ftp.gnu.org due to 
security concerns, I went through the Emacs master branch and changed 
these FTP URLs to https://ftp.gnu.org. While I was at it, I did the 
much-bigger task of changing http: to https: for most gnu.org and 
fsf.org URLs. (I ran out of energy before doing the MS-Windows files, 
and I hope someone else can take up the slack there.)

Although the web pages in question are 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.