cbm stuff from greggs pull request

libsrc/cbm510/doesclrscr.s

@@ -0,0 +1,16 @@
+;
+; 2016-06, Christian Groessler
+; 2017-07-05, Greg King
+;
+; unsigned char doesclrscrafterexit (void);
+;
+; Returns 0/1 if, after program termination, the screen isn't/is cleared.
+;
+
+        .import         return1
+
+; cc65's CBM510 programs switch to a display screen in the program RAM bank;
+; then, they switch back to the system bank when they exit.
+; The screen is cleared.
+
+        .export         _doesclrscrafterexit := return1