Pedantic fix.

Categorizing this error code as an error in the btdu tree hierarchy
caused frequent confusion (see e.g. #6 and #9).

This allows seeing exclusive/shared/distributed sizes in the browser
listing, and sorting by it.

Although there are no known situations in which this would lead to
certain data loss, after a discussion in #btrfs, general consensus
seems to be that this suggestion is suboptimal and perceived as
potentially dangerous.

Several alternatives were discussed:

- /run

  Downsides: the device name may clash with the name of a program
  which wants to create its directory in /run.  Although the mkdir
  step would fail if the directory already exists, the program might
  be started after btdu, at which point it may attempt to use the
  mounted filesystem as its temporary directory.

- /mnt

  Downsides: a practice sometimes illustrated in guides, such as in
  the Arch Linux installation guide, show mounting filesystems
  directly on /mnt.  Therefore, btdu would need to suggest mounting
  the filesystem either on /mnt directly, in which case the suggestion
  would be problematic if the user wished to scan more than one
  filesystem at once, or on a subdirectory of /mnt, in which case we
  would have to assume that there is nothing mounted on /mnt directly
  at the time when the user runs the command.

- /var/lib/btdu (or other namespaced paths)

  Downsides: the full command, though unambiguously "correct", becomes
  long and complicated enough that its intent is no longer immediately
  obvious. Given the context of the problem that we're attempting to
  solve here, providing an example for illustration is the primary
  goal, with the secondary goal of producing a working example command
  (which the user *could* copy and paste, provided that we make this
  command simple enough to be immediately understandable).

- /media

  Downsides: meant for removable media; might interfere with some
  automated removable media management software. The created directory
  may persist after reboot.

- /root

  Downsides: not a generally expected place for mount points; the
  created directory may persist after reboot; the word "root" may
  result in additional confusion, considering that the context already
  deals with filesystem roots (/) and root (top-level) subvolumes.

In the end, the approach here uses a /mnt subdirectory if /mnt itself
is not a mount point, otherwise /media if it exists, otherwise "..."
to indicate a placeholder.

Generally aim to show 3-4 decimal digits.

- Show more digits for small numeric values

  Fixes the awkwardly imprecise "1.5 GiB".
  (Now show e.g. "1.53 GiB".)

- Go to the next unit above three decimal digits

  Fixes the slightly-awkward "1010.0 GiB".
  (Now show e.g. "0.986 TiB".)

Adjust the protocol so that it's possible to distinguish received
results, regardless of what kind, between those where
BTRFS_IOC_LOGICAL_INO was called with or without
BTRFS_LOGICAL_INO_ARGS_IGNORE_OFFSET.  Previously this was only done
for ResultInodeStartMessage, but not errors.

Effectively this will now cause btdu to place some <ERROR> nodes under
the <UNREACHABLE> node, if the error was produced after retrying due
to BTRFS_IOC_LOGICAL_INO returning zero results.

Hopefully, such explanations will be more useful for btrfs experts, or
btdu users (non-btrfs-experts) attempting to diagnose their issues
with the help of a btrfs expert.

This is a pretty specific case, but it's not too difficult to
reproduce.

This is unlikely to be interesting for direct observation, but if we
want to add sorting by time, we need some meaningful way to sort items
when the selected time metric is distributed samples.

Companion to humanSize, to be used in the browser listing.

Could be used to diagnose "slow" parts of the filesystem.

Fixes #12.