dhcp: Add option appending and parsing

dhcp: Add option appending and parsing

Add functions to append and parse DHCP options. Not all options
are passed to the callback function, the ones not exposed are
pad, end, message type and overload. If indicated by the overload
option, file and sname fields will be examined for more options.

The option functions are internal to DHCP, add a new header files
for interal function prototypes.