Unix: increase the maximum number of preemptive threads
Unix: increase the maximum number of preemptive threads

We require preemptive threads for reading DNS responses on Mac.
The default maximum number of threads is quite small (4), so increase
to 1024.

Note that these threads are created on demand.

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@docker.com>

djs55/vpnkit,djs55/vpnkit,djs55/vpnkit