Added test code to configure the interface for IPv4 after DHCP.

to search through all registered option blocks.

Move some static inlines that are likely to be used frequently into
dhcpopts.c as normal functions, to save space.

is a DHCP option" data structures.

We need this code in order to be able to return a DHCP packet to a PXE NBP
which reflects options from our multiple sources (e.g. NVS and DHCP
server).  This is expensive, but necessary.  Having paid this cost, we may
as well try to use the same code to generate our DHCP request packets,
since the process is similar.

encapsulated option; this will be needed for modifications to the options
block.

multiple option sources (e.g. multiple DHCP replies, non-volatile
storage etc.)