Add indication of whether a job got killed

We have 'X' for exited with error, add 'K' for killed by
signal as well.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/HOWTO b/HOWTO
index 9263da2..662689e 100644
--- a/HOWTO
+++ b/HOWTO
@@ -1341,6 +1341,7 @@
 E		Thread exited, not reaped by main thread yet.
 _		Thread reaped, or
 X		Thread reaped, exited with an error.
+K		Thread reaped, exited due to signal.
 
 The other values are fairly self explanatory - number of threads
 currently running and doing io, rate of io since last check (read speed