nscd: don't unconditionally use PTHREAD_RWLOCK_WRITER_NONRECURSIVE_INITIALIZER_NP

PTHREAD_RWLOCK_WRITER_NONRECURSIVE_INITIALIZER_NP is Linux-only.

* nscd/connections.c (RWLOCK_INITIALIZER): Define to
PTHREAD_RWLOCK_WRITER_NONRECURSIVE_INITIALIZER_NP or
PTHREAD_RWLOCK_INITIALIZER if that is not available.
(dbs): Use RWLOCK_INITIALIZER instead of
PTHREAD_RWLOCK_WRITER_NONRECURSIVE_INITIALIZER_NP.