cp, ln, mv, install: check for vulnerable target directories

[Paul Eggert]

For years cp and friends have been subject to a symlink attack, in that 
seemingly-ordinary commands like 'cp a b' can overwrite arbitrary directories 
that the user has access to, if b's parent directory is world-writable and is 
not sticky and is manipulated by a malicious user. To help ameliorate this 
problem, I've installed the attached patch into coreutils. Although it does not 
detect every instance of this problem, the goal is that its heuristic should 
avoid false positives while catching and reporting and refusing to act on most 
instances.

As this is not an upward-compatible change, I installed this patch right after a 
coreutils release, in the hope that we will have time to shake out any problems 
with it (including reverting it entirely :-) before the next release.

0001-copy-check-for-vulnerable-target-dirs.patch
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