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

src/tool_writeout.c

@@ -131,7 +131,7 @@ void ourWriteOut(CURL *curl, struct OutStruct *outs, const char *writeinfo)
             continue;
           }
           keepit = *end;
-          *end = 0; /* zero terminate */
+          *end = 0; /* null-terminate */
           for(i = 0; variables[i].name; i++) {
             if(curl_strequal(ptr, variables[i].name)) {
               match = TRUE;