add dns_message API to add EDNS options

add dns_message API to add EDNS options

The new dns_message_ednsinit() and dns_message_ednsaddopt() functions
allow EDNS options to be added to a message one at a time; it is no
longer necessary to construct a full array of EDNS options and set
them all at once.

This allows us to simplify EDNS option handling code, and in the
future it wlil allow plugins to add EDNS options to existing
messages.