Don't call dlopen(3) for built-in NSS types - "cache", "compat",
"dns", "files", "db", and "nis". It saves some path lookups during
binary startup.

Reviewed by: markj
Approved by: re (gjb, kib)
MFC after: 2 weeks
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D17557