libfdisk: (gpt) improve and cleanup recovery code

 * use AlternativeLBA rather than hardcoded offset to backup GPT
 * create gpt_copy_header()

Signed-off-by: Karel Zak <kzak@redhat.com>
libfdisk: (gpt) allow to work with hybrid GPT

Notes:

 * fdisk don't sync hybrid MBR with GPT
 * hybrid MBR is ignored and not overwritten by PBMR
 * users is informed about expert command 'M' to toggle between MBR
   and GPT mode. It's possible to manually modify hybrid MBR.

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