NAME
        CurrentDir -- Make a directory lock the current directory

    SYNOPSIS
        oldLock = CurrentDir( lock )
        D0                    D1

        bptr currentdir(bptr)

    FUNCTION
        CurrentDir() causes a directory associated with a lock to be made
        the current directory.  The old current directory lock is returned.

        A value of zero is a valid result here, this 0 lock represents the
        root of file system that you booted from.

        Any call that has to open() or lock() files (etc) requires that
        the current directory be a valid lock or 0.

    INPUTS
        lock - BCPL pointer to a lock

    RESULTS
        oldLock - BCPL pointer to a lock

    SEE ALSO
        lock(), unlock(), open(), duplock()