init: prevent waiting forever when a setprop is incomplete.

Any app can
  socket_local_client_connect(s, "property_service", ANDROID_SOCKET_NAMESPACE_RESERVED, SOCK_STREAM)
and just keep it open without sending anything.
From then on, the property_service.c::handle_property_set_fd() will
just recv() forever.
This prevents any other properties (i.e. property_set("sys.powerctl", "reboot"))
from being processed.

Now, we just poll() for 2 sec before recv(...,MSG_DONTWAIT).

Bug: 12061560
Change-Id: Iffea4ebb444c7100b59c43ed87aecc5c99f9d3e8