change_owner().
* Change the semantics of chown(), fchown() and lchown(): when requesting a
change of the owner of a file, clear the set-user-id bit; analogous behaviour
for group changes.
* Since the above is a violation of the semantics specified by POSIX and
X/Open, add corresponding compatibility syscalls: __posix_chown(),
__posix_fchown(), __posix_lchown(). (Neither fchown() nor lchown() is
specified by POSIX; the prefix is intended to reflect the semantics.)
* Rename posix_rename() to __posix_rename() to follow the above convention.
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