detect dup EDE with bitmap and store next pos

detect dup EDE with bitmap and store next pos

In order to avoid to loop to find the next position to store an EDE in
a dns_edectx_t, add a "nextede" state which holds the next available
position.

Also, in order ot avoid to loop to find if an EDE is already existing in
a dns_edectx_t, and avoid a duplicate, use a bitmap to immediately know
if the EDE is there or not.

Those both changes applies for adding or copying EDE.

Also make the direction of dns_ede_copy more explicit/avoid errors by
making "edectx_from" a const pointer.

(cherry picked from commit 9021f9d802296241dc69431407551d824beeb814)