libfdisk: split label and label operations

libfdisk: split label and label operations

 Changes:

 - fdisk_label is a private label driver struct
 - generic header of the fdisk_label points to fdisk_label_operations
 - the private fdisk_label stuff is always allocated for all drivers
   during fdisk_context initialization

 - context->labels[] contains pointers to all supported labels (drivers)
 - context->label is a pointer to the currently used label

 This change allows to:

 - store label specific global settings (e.g. dos compatible mode,
   display units, ...) independently on the current label

 - add label specific function to the API

Signed-off-by: Karel Zak <kzak@redhat.com>