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/wherex.s

@@ -4,16 +4,23 @@
 ; unsigned char wherex (void);
 ;
 
+        .ifndef __APPLE2ENH__
+        .import         machinetype
+        .endif
+
         .export         _wherex
 
         .include        "apple2.inc"
 
 _wherex:
         lda     CH
-        .ifdef  __APPLE2ENH__
+        .ifndef __APPLE2ENH__
+        bit     machinetype
+        bpl     :+
+        .endif
         bit     RD80VID         ; In 80 column mode?
         bpl     :+
         lda     OURCH
-:       .endif
-        ldx     #>$0000
+
+:       ldx     #>$0000
         rts