lib/blkdev: add support for --lock and LOCK_BLOCK_DEVICE

lib/blkdev: add support for --lock and LOCK_BLOCK_DEVICE

All simple function to parse --lock <mode> and $LOCK_BLOCK_DEVICE,
and to flock the fd.

The supported <mode> is:

	"1" or "yes"	- LOCK_EX
	"0" or "no"	- do nothing
	"nonblock"	- LOCK_EX | LOCK_NB

The function tries LOCK_NB before the solo LOCK_EX and prints
inform user that it will wait, for example:

session A:
	# sfdisk --lock /dev/sdc

session B:
	# sfdisk --lock /dev/sdc
	sfdisk: /dev/sdc: device already locked, waiting to get lock ...
	^C

	# sfdisk --lock=nonblock /dev/sdc
	sfdisk: /dev/sdc: device already locked

Addresses: https://github.com/karelzak/util-linux/issues/921
Signed-off-by: Karel Zak <kzak@redhat.com>