[cachedhcp] Retain cached DHCPACK after startup if not already consumed

[cachedhcp] Retain cached DHCPACK after startup if not already consumed

We currently free an unclaimed cached DHCPACK immediately after
startup, in order to free up memory.  This prevents the cached DHCPACK
from being applied to a device that is created after startup, such as
a VLAN device created via the "vcreate" command.

Retain any unclaimed DHCPACK after startup to allow it to be matched
against (and applied to) any device that gets created at runtime.
Free the DHCPACK during shutdown if it still remains unclaimed, in
order to exit with memory cleanly freed.

Signed-off-by: Michael Brown <mcb30@ipxe.org>