libblkid: make uuid_is_empty() usable for whole library

Signed-off-by: Karel Zak <kzak@redhat.com>
libblkid: add functions to set PTUUID

Signed-off-by: Karel Zak <kzak@redhat.com>
libblkid: set PTUUID for GPT and DOS

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

Signed-off-by: Karel Zak <kzak@redhat.com>
libblkid: split blkid_parttable_set_id()

to avoid things like strcmp(tab->type, "gpt") in generic code.

Signed-off-by: Karel Zak <kzak@redhat.com>
libblkid: support pseudo PARTUUID= for MBR

  # blkid /dev/sdb1
  dev/sdb1: TYPE="ext2" PARTUUID="54e743a7-01"

the PARTUUID is generated from MBR Id and partition number. The same
method uses Linux kernel for root=PARTUUID= on systems with MBR.

Note that the patch also removes 0x prefix from PTUUID.

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