memdisk: handle recursive INT 13h invocation

memdisk: handle recursive INT 13h invocation

Some systems, e.g. a lot of Thinkpads, invoke INT 13h recursively from
within the stack itself.  This is generally a bad idea as it re-runs
any INT 13h hooks (including the ones that DOS installs!), but it
should work.  Keep a counter of recursive invocations and simply
bypass the whole system including the DL shift if the invocation is
recursive.

With this, MS-DOS boots on a Thinkpad T61.  FreeDOS is still broken,
though, but apparently in a different manner...

Signed-off-by: H. Peter Anvin <hpa@zytor.com>