[DHCP] Fix up fake-packet creation as used by PXENV_GET_CACHED_INFO

[DHCP] Fix up fake-packet creation as used by PXENV_GET_CACHED_INFO

Add dedicated functions create_dhcpdiscover(), create_dhcpack() and
create_proxydhcpack() for use by external code such as the PXE preboot
code.

Register ProxyDHCP options under the global scope "proxydhcp".

Unregister previously-acquired DHCP and ProxyDHCP settings when DHCP
succeeds.