bug#23033: 24.5; Lock file uses the same extension as the file it's locking

[Eli Zaretskii]

> From: Stefan Kangas <stefan@marxist.se>
> Cc: rgm@gnu.org,  23033@debbugs.gnu.org,  mgsloan@gmail.com
> Date: Fri, 08 Nov 2019 15:10:50 +0100
> 
> My suggestion would be to add a one character suffix to the file name.
> For example "."

This isn't portable enough, it won't work on MS-Windows, where an
empty extension is "normalized" by the file-name APIs in a way that
removes the last dot, so you get the file name without the last dot.