Add NOFOLLOW flag to set-file-modes etc.

[Paul Eggert]

Bug#39683 proposes a patch to add an optional nofollow argument to 
set-file-modes and related functions; if this flag is t, the functions 
do not follow symbolic links. This should help Emacs avoid race 
conditions where it mistakenly changes the permissions of an arbitrary 
victim file that happens to be the target of a recently-created symlink.

Since the implementation of this new argument uses the POSIX fchmodat 
function with the AT_SYMLINK_NOFOLLOW flag, some work may be needed in 
the Microsoft porting code to emulate fchmodat.

Also, packages like Tramp that have a file-modes or set-file-modes 
wrapper should be updated if possible to support the new optional flag 
if it is given. For now, the patch leaves these as FIXME comments.

This is designed to be an upward-compatible change, even though the 
default, traditional behavior of following the symbolic link is 
typically more dangerous than the new option of not following the link.

Comments welcome.

https://debbugs.gnu.org/39683