collation-related loose ends before beta2

Status on collation loose ends:

1. There's an open item "Switch to ICU for 17". It's a little bit
confusing exactly what that means, and the CF entry refers to two
items, one of which is the build-time default to --with-icu. As far as
I know, building with ICU by default is a settled issue with no
objections. The second issue is the initdb default, which is covered by
the other open item. So I will just close that open item unless someone
thinks I'm missing something.

2. Open item about the unfriendly rules for choosing an ICU locale at
initdb time. Tom, Robert, and Daniel Verite have expressed concerns
(and at least one objection) to initdb defaulting to icu for --locale-
provider. Some of the problems have been addressed, but the issue about
C and C.UTF-8 locales is not settled. Even if it were settled I'm not
sure we'd have a clear consensus on all the details. I don't think this
should proceed to beta2 in this state, so I intend to revert back to
libc as the default for initdb. [ I believe we do have a general
consensus that ICU is better, but we can signal it other ways: through
documentation, packaging, etc. ]

3. The ICU conversion from "C" to "en-US-u-va-posix": cut out this code
(it was a small part of a larger change). It's only purpose was
consistency between ICU versions, and nobody liked it. It's only here
right now to avoid test failures due to an order-of-commits issue; but
if the initdb default goes back to libc it won't matter and I can
remove it.

4. icu_validation_level WARNING or ERROR: right now an invalid ICU
locale raises a WARNING, but Peter Eisentraut would prefer an ERROR.
I'm still inclined to leave it as a WARNING for one release and
increase it to ERROR later. But if the default collation provider goes
back to libc, the risk of ICU validation errors goes way down, so I
don't object if Peter would like to change it back to an ERROR.

Regards,
Jeff Davis