First-draft release notes for 17.1.

First-draft release notes for 17.1.

(We lack a query for identifying broken foreign keys in the first
changelog item, but the rest of this is in reviewable shape.)

As usual, the release notes for other branches will be made by cutting
these down, but put them up for community review first.

Also as usual for a .1 release, there are some entries here that
are not really relevant for v17 because they already appeared in 17.0.
Those'll be removed later.