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/mtchline.s

@@ -0,0 +1,36 @@
+;
+; Ullrich von Bassewitz, 08.08.1998
+; Colin Leroy-Mira, 26.05.2025
+;
+; void __fastcall__ mt_chlinexy (unsigned char x, unsigned char y, unsigned char length);
+; void __fastcall__ mt_chline (unsigned char length);
+;
+
+.ifdef  __APPLE2ENH__
+
+        .export         _mt_chlinexy, _mt_chline, chlinedirect
+        .import         gotoxy, cputdirect
+
+        .include        "zeropage.inc"
+        .include        "apple2.inc"
+
+_mt_chlinexy:
+        pha                     ; Save the length
+        jsr     gotoxy          ; Call this one, will pop params
+        pla                     ; Restore the length and run into _chline
+
+_mt_chline:
+        ldx     #'_' | $80      ; Underscore, screen code
+
+chlinedirect:
+        stx     tmp1
+        cmp     #$00            ; Is the length zero?
+        beq     done            ; Jump if done
+        sta     tmp2
+:       lda     tmp1            ; Screen code
+        jsr     cputdirect      ; Direct output
+        dec     tmp2
+        bne     :-
+done:   rts
+
+.endif