Like ICU, allow -1 length to mean that the input string is NUL-
terminated for pg_strncoll(), pg_strnxfrm(), and pg_strnxfrm_prefix().
This simplifies the API and code a bit.
Along with some other refactoring in this area, we are getting close to
the point where the collation provider can just be a table of methods,
which means we can add an extension hook to provide a different method
table. That still requires more work, I'm just mentioning it here for
context.
Regards,
Jeff Davis