fdisk: use remove read_int() usage from fdisk.c

Signed-off-by: Karel Zak <kzak@redhat.com>
fdisk: use new menu infrastructure to verify keys

 - use generic function to ask for key
 - verify the key against the current menu
 - call menu callback if defined

Signed-off-by: Karel Zak <kzak@redhat.com>
fdisk: add GPT specific callback to menu

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

Currently we use private libfdisk in fdisk code, because the library
API is not stable yet. Let's add debug flag to make it obvious in the
debug output what is from libfdisk and what is from fdisk frontend.

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