Eliminate conf_lock and instead rely on the NSS write lock to protect
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