Updated terminology in docs, comments and phrases to refer to C strings
as "null-terminated". Done to unify with how most other C oriented docs
refer of them and what users in general seem to prefer (based on a
single highly unscientific poll on twitter).
Reported-by: coinhubs on github
Fixes#5598Closes#5608
This repeats what has already been documented in both the curl manpage
and CURLOPT_USERPWD documentation but is provided here for completeness
as someone may not especially read the latter when using libcurl.
