SAFE_MACROS: Add chmod() and fchmod()

Signed-off-by: Cyril Hrubis <chrubis@suse.cz>
syscalls/link: Cleanup.

* Cleanup excessive comments

* Make use of SAFE_MACROS()

* Check return values from all syscalls

* Removed obsolete CRAY specific test

* Add tst_require_root() to link06 and link07

* ...

Signed-off-by: Cyril Hrubis <chrubis@suse.cz>