btrfs-progs: convert: allow to set filesystem features

btrfs-progs: convert: allow to set filesystem features

Curretnly there are no features set for the converted filesystem and
it's not possible to request it like in mkfs.

Add new option -O|--features, use -O list-all to show all that are
supported for convert.

Note: from now on, convert without any -O option will use the same
defaults as mkfs. The original behaviour was rather confusing.

Convert is now more verbose about the parameters of the btrfs filesytem.

Signed-off-by: David Sterba <dsterba@suse.cz>