First-draft release notes for 9.4.1 et al.

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

Note: a significant fraction of these items don't apply to 9.4.1, only to
older branches, because the fixes already appeared in 9.4.0.  These can be
distinguished by noting the branch commits in the associated SGML comments.
This will be adjusted tomorrow while copying items into the older
release-X.Y.sgml files.  In a few cases I've made two separate entries with
different wordings for 9.4 than for the equivalent commits in the older
branches.