Revert "Don't truncate database and user names in startup packets."

Revert "Don't truncate database and user names in startup packets."

This reverts commit 562bee0fc13dc95710b8db6a48edad2f3d052f2e.

We received a report from the field about this change in behavior,
so it seems best to revert this commit and to add proper
multibyte-aware truncation as a follow-up exercise.

Fixes bug #18711.

Reported-by: Adam Rauch
Reviewed-by: Tom Lane, Bertrand Drouvot, Bruce Momjian, Thomas Munro
Discussion: https://postgr.es/m/18711-7503ee3e449d2c47%40postgresql.org
Backpatch-through: 17