- Use _SEH2_YIELD when returning from an exception instead of returning outside the...

- Use _SEH2_YIELD when returning from an exception instead of returning outside the SEH block. Avoids unnecessary status checks for the most common case (no exception). Move the cleanup code into the handler too in favor of the no-exception case. Futhermore, don't call ExSystemExceptionFilter when we know we are called from user mode. Finally, only enter SEH if we need to do any probing.
- Re-enable user mode probes in KiRaiseException; they do not seem to be an issue anymore -- booting and running the ntdll exception Winetest didn't reveal any issue. Put a breakpoint there in case this code is ever hit (unlikely).

svn path=/trunk/; revision=42923