partrt: create sub-command minor bug fixes

The command will now create the restore file correctly so it can be read by
undo.

Some print estetic changes.

Signed-off-by: Mats Liljegren <mats.liljegren@enea.com>
partrt: More verbose fail() message

Now prints file and line when fail() is called.

Signed-off-by: Mats Liljegren <mats.liljegren@enea.com>
partrt: Grammatic fix of fail() message

Signed-off-by: Mats Liljegren <mats.liljegren@enea.com>
partrt: Mockup of file operations

This is a first step into having mockups for file operations. More work is
needed on fopenat() and fscanf(), among others.

Signed-off-by: Mats Liljegren <mats.liljegren@enea.com>