fix issue #144 by correcting getbpb return code

getbpb now returns 0 instead of S_DONE in case of an uninitialized
partition, copying the default BPB into the BPB.

The previous return of S_DONE in case of uninitialized partitions
resulted in rp->r_bpptr not getting set in bldbpb. This in
combination with indicating success resulted in garbage returned
via rp->r_bpptr.

The DPB values are now being set to the default BPB ones in media_check
https://github.com/FDOS/kernel/blob/ea951d8136444e334f06a313465b5844f738354e/kernel/fatfs.c#L1728
via call to bpb_to_dpb in case of an uninitialized partition.
This may have side effects. But because DF_NOACCESS is still set, I
think it is the right way to do it.

The commit also masks high bit of AL for INT25/26 containing the drive
number. Some programs may set the bit according to RBIL:

"examination of CPWIN386.CPL indicates that if this call fails with
error 0408h on an old-style (<32M) call, one should retry the
call with the high bit of the drive number in AL set"

Leaving the bit set may render the given drive number unusable.
It should do no harm to mask it to increase the chance of the operation
to succeed. Also, the AH should be set to zero, because drive is given
only in AL.

kernel/dsk.c

@@ -392,7 +392,7 @@ STATIC WORD getbpb(ddt * pddt)
   {
     /* copy default bpb to be sure that there is no bogus data */
     memcpy(pbpbarray, &pddt->ddt_defbpb, sizeof(bpb));
-    return S_DONE;
+    return 0;
   }
 
   pddt->ddt_descflags &= ~DF_NOACCESS;  /* set drive to accessible */

kernel/inthndlr.c

@@ -1820,7 +1820,9 @@ VOID ASMCFUNC int2526_handler(WORD mode, struct int25regs FAR * r)
   else
     mode = DSKREADINT25;
 
-  drv = r->ax;
+  drv = r->ax & 0x7f; /* according to RBIL, some programs may try with */
+                      /* high bit of AL set, so mask it together with AH */
+                      /* otherwise we might access a non-existing unit */
 
   if (drv >= lastdrive)
   {