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/machinetype.s

@@ -0,0 +1,24 @@
+.ifndef __APPLE2ENH__
+
+        .constructor    initmachinetype, 8
+
+        .import         ostype
+        .export         machinetype
+
+        .segment        "ONCE"
+
+initmachinetype:
+        ldx     ostype
+        cpx     #$31          ; Apple //e enhanced?
+        ror     machinetype   ; Carry to high bit
+        cpx     #$30          ; Apple //e?
+        ror     machinetype
+        rts
+
+        .data
+
+; bit 7: Machine is a //e or newer
+; bit 6: Machine is a //e enhanced or newer
+machinetype:            .byte 0
+
+.endif