NSS configuration state.
As a side effect, this fixes a race condition which can occur if multiple
threads call nsdispatch(3) concurrently before nsswitch.conf has been
parsed. Previously, the thread holding conf_lock could cause other threads
to return from nss_configure() before nsswitch.conf had been parsed, forcing
them to fall back to the default sources for their NSS methods.
Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D994
MFC after: 1 month
Sponsored by: EMC / Isilon Storage Division
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