Apple2: implement get_tv()

get_tv() will return TV_NTSC or TV_PAL for any Apple II model
with a way of checking vblank. For Apple ][ and ][+ it will
return TV_OTHER and let the user figure it out in another way.

doc/apple2enh.sgml

@@ -333,6 +333,7 @@ usage.
 <item>_datetime
 <item>beep
 <item>dir_entry_count
+<item>get_tv
 <item>get_ostype
 <item>gmtime_dt
 <item>mktime_dt