[pxe] Ensure cached DHCPACK is retrieved prior to network device creation

The retrieval of the cached DHCPACK and the creation of network
devices are both currently scheduled as STARTUP_NORMAL.  It is
therefore possible that the cached DHCPACK will not be retrieved in
time for cachedhcp_probe() to apply it to the relevant network device.

Fix by retrieving the cached DHCPACK at initialisation time rather
than at startup time.

As an optimisation, an unclaimed cached DHCPACK can be freed
immediately after the last network device has been created, rather
than waiting until shutdown.

Reported-by: Espen Braastad <espen.braastad@redpill-linpro.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>