Allow length=-1 for NUL-terminated input to pg_strncoll(), etc.
Like ICU, allow a length of -1 to be specified for NUL-terminated
arguments to pg_strncoll(), pg_strnxfrm(), and pg_strnxfrm_prefix().
Simplifies the code and comments.
Discussion: https://postgr.es/m/
2d758e07dff26bcc7cbe2aec57431329bfe3679a.camel@j-davis.com