btrfs-progs: btrfstune: add '-U' and '-u' option to change fsid

btrfs-progs: btrfstune: add '-U' and '-u' option to change fsid

Add two options, '-U' and '-u' for btrfstune.

For '-u', it is used to change fsid to a random new UUID.
For '-U', it is used to change fsid to a specified UUID.

Both will also change the internal use only chunk_tree_uuid to a random
new UUID.

Although there is a GNU getopt extension "::" to get optional
option-argument, but is forbidden by POSIX.1-2008, so use split options
here.

Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.cz>