libfdisk: add {first,last}-lba header to sfdisk scritps

libfdisk: add {first,last}-lba header to sfdisk scritps

The current sfdisk does not allow to create partition that
starts before the default libfdisk First LBA (~1MiB). It
means that

 # sfdisk --dump /dev/sda > foo
 # sfdisk /dev/sdb < foo

does not work on systems where 1st partition does not start at offset
2048.

This patch add new headers to scripts to inform libfdisk about different
First/Last LBA ranges. For example:

 label: gpt
 first-lba: 34

allows to override the library default.

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