Use sock->nchildren instead of mgr->nworkers when initializing NM

Use sock->nchildren instead of mgr->nworkers when initializing NM

On Windows, we were limiting the number of listening children to just 1,
but we were then iterating on mgr->nworkers.  That lead to scheduling
more async_*listen() than actually allocated and out-of-bound read-write
operation on the heap.

(cherry picked from commit 87c5867202935c59dfe66321238275ba4a953b53)