dynbuf: make *addf() not require extra mallocs

... by introducing a printf() function that appends directly into a
dynbuf: Curl_dyn_vprintf(). This avoids the mandatory extra malloc so if
the buffer is already big enough it can just printf directly into it.

Since this less-malloc version requires tthe use of a library internal
printf function, we only provide this version when building libcurl and
not for the dynbuf code that is used when building the curl tool.

Closes #5998

lib/dynbuf.h

@@ -62,6 +62,10 @@ char *Curl_dyn_ptr(const struct dynbuf *s);
 unsigned char *Curl_dyn_uptr(const struct dynbuf *s);
 size_t Curl_dyn_len(const struct dynbuf *s);
 
+/* returns 0 on success, -1 on error */
+/* The implementation of this function exists in mprintf.c */
+int Curl_dyn_vprintf(struct dynbuf *dyn, const char *format, va_list ap_save);
+
 /* Dynamic buffer max sizes */
 #define DYN_DOH_RESPONSE    3000
 #define DYN_DOH_CNAME       256