kvs: replace list of message with msglist

[chu11]

a misc cleanups PR ... mostly wanted to pick off cleanups from multiple branches/PRs/WIPs.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 83.35%. Comparing base (648306d) to head (e2e8a96).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #6047 +/- ## ========================================== + Coverage 83.34% 83.35% +0.01% ========================================== Files 519 519 Lines 83746 83735 -11 ========================================== + Hits 69795 69797 +2 + Misses 13951 13938 -13 ``` | [Files](https://app.codecov.io/gh/flux-framework/flux-core/pull/6047?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=flux-framework) | Coverage Δ | | |---|---|---| | [src/modules/kvs/kvsroot.c](https://app.codecov.io/gh/flux-framework/flux-core/pull/6047?src=pr&el=tree&filepath=src%2Fmodules%2Fkvs%2Fkvsroot.c&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=flux-framework#diff-c3JjL21vZHVsZXMva3ZzL2t2c3Jvb3QuYw==) | `69.85% <100.00%> (ø)` | | | [src/modules/kvs/kvs.c](https://app.codecov.io/gh/flux-framework/flux-core/pull/6047?src=pr&el=tree&filepath=src%2Fmodules%2Fkvs%2Fkvs.c&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=flux-framework#diff-c3JjL21vZHVsZXMva3ZzL2t2cy5j) | `71.35% <80.00%> (+0.19%)` | :arrow_up: | ... and [16 files with indirect coverage changes](https://app.codecov.io/gh/flux-framework/flux-core/pull/6047/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=flux-framework)