bug#33113: incorrect and inconsistent quoting in ls output

[Paul Eggert]

Vincent Lefevre wrote:
> > Now we could quote with -F only if these chars are at the end,
> > though I'm not sure that complexity is warranted.
> In any case, this form of quoting incorrect with -b, as \= is invalid
> in ISO C.

That's right, we need another way to escape classifier characters with -bF, 
since the current method is clearly wrong.

Let's use ""= instead, as it's valid ISO C. In other words, where we currently 
do this:

$ python -c "import socket as s; sock = s.socket(s.AF_UNIX); sock.bind('b')"
$ touch b= b=x
$ ls -bF
b=  b\=  b\=x

the last command should output this instead:

b=  b""=  b=x

This works because in ISO C "b""=" is equivalent to "b=". We should do this only 
with characters at the end, because it's not needed elsewhere and the "" is 
annoying.