* Use 0/1 instead of sysexits. Man pages are confusing on this topic,
but 0/1 is sufficient for nvmecontrol.
* Use err function family where possible instead of fprintf/exit.
* Fix some typing errors.
* Clean up some error message inconsistencies.
Sponsored by: Intel
Submitted by: bde (parts of firmware.c changes)
MFC after: 3 days