The SysV ABI says that the DF flag should be clear on function entry.
That means we have to clear it when jumping into the kernel from some
random userspace context.
Added the exception_code field to RegisterDump, removing the need
for RegisterDumpWithExceptionCode. To accomplish this, I had to
push a dummy exception code during some interrupt entries to properly
pad out the RegisterDump. Note that we also needed to change some code
in sys$sigreturn to deal with the new RegisterDump layout.
