Apple2: implement sleep using MONWAIT

Also publish detect_iigs(), set_iigs_speed() and get_iigs_speed(). Refactor to only store one ostype variable.

libsrc/apple2/get_iigs_speed.s

@@ -0,0 +1,22 @@
+;
+; Colin Leroy-Mira <colin@colino.net>, 2024
+;
+; unsigned char __fastcall__ get_iigs_speed(void)
+;
+
+        .export         _get_iigs_speed
+        .import         ostype, return0
+
+        .include        "apple2.inc"
+        .include        "accelerator.inc"
+
+_get_iigs_speed:
+        lda     ostype          ; Return SLOW if not IIgs
+        bpl     :+
+        lda     CYAREG          ; Check current setting
+        bpl     :+
+        lda     #SPEED_FAST
+        ldx     #$00
+        rts
+        .assert SPEED_SLOW = 0, error
+:       jmp     return0         ; SPEED_SLOW