Add Atari version of of doesclrscrafterexit().

- Update documentation.
- Update atari.h and apple2.h header files.
- Adapt Atari test/target programs.
- Fix a typo in "div" entry in funcref.sgml.

libsrc/atari/doesclrscr.s

@@ -0,0 +1,19 @@
+;
+; Christian Groessler, June-2016
+;
+; unsigned char doesclrscr(void);
+;
+; returns 0/1 if after program termination the screen isn't/is cleared
+;
+
+        .export  _doesclrscrafterexit
+        .import  __dos_type
+        .include "atari.inc"
+
+_doesclrscrafterexit:
+        ldx     #0
+        lda     __dos_type
+        cmp     #MAX_DOS_WITH_CMDLINE + 1
+        txa
+        rol     a
+        rts