Make the DOSVER function call return 6Fh in C in DOS 1 mode

The MSX-DOS 2 documentation says that DOSVER in DOS 1 returns B=0
and C undefined, but the fact is that in MSX-DOS 1 register C is
preserved (so 6Fh) and seems like some programs rely on that fact
to detect the DOS 1 mode. Nextor was corrupting C in this case,
so even if strictly speaking that shouldn't be an issue, this commit
changes the behavior of DOSVER in DOS 1 to return C=6Fh.

source/kernel/bank4/partit.mac

@@ -4169,7 +4169,7 @@ IDRVR_RET:
 
 DOSV1_2:
 	xor	a
-	ld	b,a
+	ld	bc,_DOSVER##  ;Make B=0 and preserve C, some programs actually check it
 	ret