Register nss_atexit() before parsing nsswitch.conf for the first time.

NSS modules are loaded when nsswitch.conf is parsed and may register their
own atexit handlers with libc. nss_atexit() unloads any dynamically loaded
NSS modules, so it should run only after the modules' atexit handlers have
been invoked.

MFC after: 2 weeks
Sponsored by: Dell EMC Isilon