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.

doc/apple2.sgml

@@ -361,6 +361,7 @@ usage.
 <item>rebootafterexit
 <item>ser_apple2_slot
 <item>tgi_apple2_mix
+<item>videomode
 </itemize>
 
 
@@ -406,6 +407,10 @@ The names in the parentheses denote the symbols to be used for static linking of
   with <tt/-S $4000/ to reserve the first hires page or with <tt/-S $6000/
   to reserve both hires pages.
 
+  Note that the second hires page is only available if the text display is not in
+  80 column mode. This can be asserted by calling <tt/videomode (VIDEOMODE_40COL);/
+  before installing the driver.
+
   The function <tt/tgi_apple2_mix()/ allows to activate 4 lines of text. The
   function doesn't clear the corresponding area at the bottom of the screen.