Apple2: Make 80-columns support dynamic on apple2 target

Add a machinetype identifier to help us quickly identify
Apple //e (bit 7) and //e enhanced (bit 6).

Use it in conio functions for 80-columns code instead of
relying entirely on the __APPLE2ENH__ target.

Move videomode() to the apple2 target, and have it return
an error if 80-columns hardware is not available - this
is a lie for now, it is considered available on //e enhanced,
which may not be true, and not available on //e, which
may also be not true. An ulterior patch will make that
check correctly.

Adapt the box/line drawing characters so that one can use
MouseText on the apple2 target if it is available, by
defining DYN_DRAW_BOX. No change by default: MouseText is
considered available on apple2enh and not available on
apple2.

libsrc/apple2/cgetc.s

@@ -7,6 +7,10 @@
 ;
 
         .export         _cgetc
+
+        .ifndef __APPLE2ENH__
+        .import         machinetype
+        .endif
         .import         cursor, putchardirect
 
         .include        "zeropage.inc"
@@ -18,11 +22,14 @@ _cgetc:
         beq     :+
 
         ; Show caret.
-        .ifdef  __APPLE2ENH__
-        lda     #$7F | $80      ; Checkerboard, screen code
-        .else
+        .ifndef __APPLE2ENH__
         lda     #' ' | $40      ; Blank, flashing
+        bit     machinetype
+        bpl     put_caret
         .endif
+
+        lda     #$7F | $80      ; Checkerboard, screen code
+put_caret:
         jsr     putchardirect   ; Saves old character in tmp3
 
         ; Wait for keyboard strobe.