Currently syscall_badsys is called to handle faults when reading the
sixth parameter in sysenter; however that routine assumes that
registers have already been pushed on the stack, and this is not the
case (in other words, it will currently try to pop beyond the end of
the thread stack).

This patch adds a new "function", syscall_fault, that saves register
and returns.

The return value is changed to EFAULT, which seems more appropriate
than ENOSYS.