[$] 5.15 Merge window, part 1

As of this writing, 3,440 non-merge changesets have been pulled into the
mainline repository for the 5.15 development cycle. A mere 3,440 patches
may seem like a slow start, but those patches are densely populated with
significant new features. Read on for a look at what the first part of the
5.15 merge window has brought.