Move update_sigs() to lib/dns/zone_p.h

Move update_sigs() to lib/dns/zone_p.h

Rename update_sigs() to dns__zone_updatesigs() and move it to
lib/dns/zone_p.h, so that it can be unit tested.  Add a comment
describing the purpose of this function.

(cherry picked from commit b1947cee8210844b917320453cb308fbe168d320)
(cherry picked from commit 4d06f50ba84edca141c28f9decd6640465c5f9ed)
(cherry picked from commit 5c71a025c6ec1cbaab04ada54101200a48a98bd1)
(cherry picked from commit ae03a5c1fa5d7d6cce93273cd41171595882c7d4)