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/dynbuf.c

@@ -161,7 +161,7 @@ CURLcode Curl_dyn_addn(struct dynbuf *s, const void *mem, size_t len)
 }
 
 /*
- * Append a zero terminated string at the end.
+ * Append a null-terminated string at the end.
  */
 CURLcode Curl_dyn_add(struct dynbuf *s, const char *str)
 {