Improve flexibility of ioat_test / ioatcontrol(8)

The test logic now preallocates memory before running the test.

The buffer size is now configurable. Post-copy verification is
configurable. The number of copies to chain into one transaction (one
interrupt) is configurable.

A 'duration' mode is added, which repeats the test until the duration
has elapsed, reporting the B/s and transactions completed.

ioatcontrol.8 has been updated to document the new arguments.

Initial limits (on this particular Broadwell-DE) (and when the
interrupts are working) seem to be: 256 interrupts/sec or ~6 GB/s,
whichever limit is more restrictive.

Unfortunately, it seems the interrupt-reset handling on Broadwell isn't
working as intended. That will be fixed in a later commit.

Sponsored by: EMC / Isilon Storage Division