dump: add API to write header of flatten format

dump: add API to write header of flatten format

flatten format will be used when writing kdump-compressed format. The format is
also used by makedumpfile, you can refer to the following URL to get more
detailed information about flatten format of kdump-compressed format:
http://sourceforge.net/projects/makedumpfile/

The two functions here are used to write start flat header and end flat header
to vmcore, and they will be called later when flatten format is used.

struct MakedumpfileHeader stored at the head of vmcore is used to indicate the
vmcore is in flatten format.

struct MakedumpfileHeader {
    char signature[16];     /* = "makedumpfile" */
    int64_t type;           /* = 1 */
    int64_t version;        /* = 1 */
};

And struct MakedumpfileDataHeader, with offset and buf_size set to -1, is used
to indicate the end of vmcore in flatten format.

struct MakedumpfileDataHeader {
    int64_t offset;         /* = -1 */
    int64_t buf_size;       /* = -1 */
};

Signed-off-by: Qiao Nuohan <qiaonuohan@cn.fujitsu.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>