Turn the CPU burner into a real io engine

This removes the special casing in fio.c for the cpu engine, and
also gets rid of FIO_CPUIO.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/HOWTO b/HOWTO
index d1a3ad1..1374d05 100644
--- a/HOWTO
+++ b/HOWTO
@@ -301,6 +301,12 @@
 				or receive, if the latter only the port
 				argument is used.
 
+			cpu	Doesn't transfer any data, but burns CPU
+				cycles according to the cpuload= and
+				cpucycle= options. Setting cpuload=85
+				will cause that job to do nothing but burn
+				85% of the CPU.
+
 			external Prefix to specify loading an external
 				IO engine object file. Append the engine
 				filename, eg ioengine=external:/tmp/foo.o