bug#33564: Incorrect path canonicalisation

[Mattias Andrée]

Emacs 26.1 removes ..'s incorrectly from file names.
Emacs removes the directory in front the .. rather than
getting its parent directory.

Example:

        cd
        mkdir -p 1/2
        cd 1/2
        echo 3 > ../../3
        ln -s ~ 4
        echo 5 > 5
        emacs 4/../5 # works, but shouldn't
        emacs 4/../$USER/3 # does not works, should

On Linux, the proper way to get the canonical path
for a file with the file descriptor $fd:

        stat(3) /dev/fd/$fd
        p := readlink(3) /dev/fd/$fd
        if (st_nlinks != 0) {
                stat(3) /dev/fd/$fd
                if (st_nlinks == 0) {
                        p := readlink(3) /dev/fd/$fd
                        remove " (deleted)" from the end of p
                }
        } else {
                remove " (deleted)" from the end of p
        }
        canonical path is p