getspnam.3: ATTRIBUTES: Note functions that are/aren't thread-safe

getspnam.3: ATTRIBUTES: Note functions that are/aren't thread-safe

After research, We think
* getspnam(),
* getspent(),
* setspent(),
* endspent(),
* getspent_r(),
* fgetspent(),
* sgetspent(),
are not thread-safe. And
* putspent(),
* getspnam_r(),
* sgetspent_r(),
* lckpwdf(),
* ulckpwdf(),
* fgetspent_r(),
are thread-safe. But, there are not
markings of them in glibc document.

Signed-off-by: Zeng Linggang <zenglg.jy@cn.fujitsu.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>