target: Add register_type and preempt_type enums to clarify code

core_scsi3_enulate_pro_register took an 'ignore_key' parameter that
really distinguished between REGISTER and REGISTER_AND_IGNORE_EXISTING_KEY
registration types, which was a little confusing. Same situation for
PREEMPT and PREEMPT_AND_ABORT. Use enums to add a little more
descriptiveness to the code.
Signed-off-by: Andy Grover <agrover@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>