fgetpwent.3: ATTRIBUTES: Note function that is not thread-safe

fgetpwent.3: ATTRIBUTES: Note function that is not thread-safe

The marking matches glibc marking.
marking of function in glibc is:
- fgetgrent: MT-Unsafe race:fpwent

ps: We think race:fpwent in glibc maybe hard for users to understand,
and have sent a patch to the GNU libc community for changing it to
race:fgetpwent, however, something about the copyright impeded the progress.
Here we mark it "race:fgetpwent", so there is a little different.

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