btrfs-progs: subvol: change set-default to also accept path

btrfs-progs: subvol: change set-default to also accept path

This patch changes "subvol set-default" to also accept the subvolume path
for convenience.

If there are two args, they are assumed as subvol id and path to the fs
(the same as current behavior), and if there is only one arg, it is assumed
as the path to the subvolume.

subvol id is resolved by test_issubvolume() + lookup_path_rootid().
The empty subvol (ino == 2) will get error on test_issubvolume() which
checks whether inode num is 256 or not.

Issue: #35
Signed-off-by: Tomohiro Misono <misono.tomohiro@jp.fujitsu.com>
[ update documentation, use the new multi-line command scheme ]
Signed-off-by: David Sterba <dsterba@suse.com>