syscalls: Rewrite readlinkat01

* Adds configure check for readlinkat and lapi/readlinkat.h header

* Fixes the mess in readlinkat01 testcase
  - Cleanup
  - Use tst_exit() instead of return 0
    (the test failure wasn't propagated out at all!)
  - Make use of tst_tmpdir() and SAFE_MACROS()
  - ...

Signed-off-by: Cyril Hrubis <chrubis@suse.cz>
readlinkat/readlinkat02.c: add new errno testes

Add EINVAL and ENOTDIR errno testes for readlinkat(2)

Signed-off-by: Zeng Linggang <zenglg.jy@cn.fujitsu.com>