bug#33366: emacsclient to not follow symlinks to sockets

[Paul Eggert]

After looking into this, I don't see any good solution on older systems where 
Emacs sockets are put under /tmp, particularly on ancient systems where /tmp is 
not sticky. There are too many ways to fool Emacs with symlinks, and none of the 
workarounds avoid all races. I did install the attached patch to fix a race that 
is reasonably easy to fix, by checking the socket's UID after connecting to it 
as well as before connecting to it.

On more-modern desktops where Emacs sockets live under XDG_RUNTIME_DIR (see 
Bug#33367, now fixed), the security issues of symlinks-to-sockets should go away 
unless the user explicitly specifies a socket. This may be the best we can do.

0001-emacsclient-fix-symlink-socket-race.patch
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