ctl: add a new error type to distinguish EINVAL vs. EEXIST

[kevans]

Summary

For port creation requests, CTL_LUN_ERROR will almost always effectively
be EINVAL, with one exception: attempting to add a port that already
exists.

Add a new error type to distinguish between the two, which maintains
the same semantics (setting the error string) for applications that
don't actually care about the differnce between the error types.

This will be used to surface an EEXIST in ctld so that we can proceed
with updating the port.