Check -U option for unknown memories during parsing

$ avrdude -qp ATmega2560 -c usbtiny -U flesh:w:blink-mega2560+lext-test.hex:i
avrdude: unknown memory type flesh
avrdude: error parsing update operation 'flesh:w:blink-mega2560+lext-test.hex:i'

src/avr.c

@@ -1253,7 +1253,7 @@ void avr_add_mem_order(const char *str) {
   exit(1);
 }
 
-int avr_known_mem(const char *str) {
+int avr_mem_is_known(const char *str) {
   for(size_t i=0; i < sizeof avr_mem_order/sizeof *avr_mem_order; i++)
     if(avr_mem_order[i] && !strcmp(avr_mem_order[i], str))
       return 1;