fio: add multi directory support

fio: add multi directory support

This patch adds support for ':' seperated multiple directories at the
directory config statement in order to achieve an automatic distribution
of job clones (numjob) across directories.

That way people can distribute a load across these directories (usually
mount points of disks) automatically - changing numjob will be
sufficient to get all job clones evenly (optimal if dirs % numjobs = 0,
otherwise as good as possible) distributed at all times.

To avoid confused users old config Files will behave like they always
did, old fio binaries using new config files won't abort but just use
the first specified dir. If one specifies an explcit (non generated)
filename the distribution to many directories is also deactivated.

It also fixes an issue of events seeming out of order like when running
with --debug=file seeing the "..." message meaning "I created the
clones" prior to the last clone activities.  Now the clones are called
with index N-1 .. 1, zero being the base thread as before.

Signed-off-by: Christian Ehrhardt <ehrhardt@linux.vnet.ibm.com>
Signed-off-by: Jens Axboe <axboe@fb.com>