command line) and the device path (what we passed to open()). Use
the former in diagnostics.
- when adding or removing partitions, print a single line to stdout for
each partition that was added or removed, indicating its name.
- add an -a option to 'gpt remove' which must be explicitly specified
to remove all partitions.
Approved by: marcel (in prinicple)
MFC after: 2 weeks