terminology: call them null-terminated strings

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 #5598
Closes #5608

lib/strerror.c

@@ -795,7 +795,7 @@ const char *Curl_strerror(int err, char *buf, size_t buflen)
 
 #endif /* end of not Windows */
 
-  buf[max] = '\0'; /* make sure the string is zero terminated */
+  buf[max] = '\0'; /* make sure the string is null-terminated */
 
   /* strip trailing '\r\n' or '\n'. */
   p = strrchr(buf, '\n');