Switch from __nss_next API to __nss_next2 API
__nss_passwd_lookup2, __nss_group_lookup2, and __nss_next2 were added
in glibc 2.8.
__nss_passwd_lookup, __nss_group_lookup, and __nss_next were removed
in glibc 2.27.
Signed-off-by: Anders Kaseorg <andersk@mit.edu>