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
To support telling a string is nul-terminated, symbol CURL_ZERO_TERMINATED
has been introduced.
Documentation updated accordingly.
symbols in versions updated. Added form API symbols deprecation info.