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/http.c

@@ -270,7 +270,7 @@ char *Curl_copy_header_value(const char *header)
     return NULL;
 
   memcpy(value, start, len);
-  value[len] = 0; /* zero terminate */
+  value[len] = 0; /* null-terminate */
 
   return value;
 }