btrfs-progs: subvol list: use libbtrfsutil

btrfs subvol list has its own subvolume walking implementation that we
can replace with a libbtrfsutil subvolume iterator. Most of the changed
lines are removing the old implementation and mechanically updating the
comparators, filters, and printers to use libbtrfsutil's subvolume info.
The interesting parts are:

1. We can replace the red-black tree of subvolumes with an array that we
   qsort.
2. Listing deleted subvolumes needs a different codepath, but we don't
   need a filter for it anymore.
3. We need some hacks to maintain the weird path behavior documented in
   the previous commit.

In addition to removing a bunch of redundant code, this also prepares us
for allowing subvol list by unprivileged users in some cases.

Signed-off-by: Omar Sandoval <[email protected]>