ctladm: Fix a race when loading ctl.ko

If multiple ctladm processes try to load ctl.ko at the same time, only
one will succeed. Handle this possibility by retrying the operation
(open /dev/cam/ctl) if kldload returns EEXIST, rather than bailing.
This at least helps ensure that ctladm tests can be run in parallel
when ctl.ko is not preloaded.

Reviewed by: asomers
MFC after: 1 week