Closes #41

Closes #39

This brings a massive speedup: 58.768s -> 2.010s on my sbuild disk.
We spend most of time zeroing a buffer to read files with 1-2 extents.

Noticing this is Zygo Blaxell's fault: praise him with great praise!

Closes: #30

Patch by Helmut Grohne.

_FILE_OFFSET_BITS is already set to 64

Fixes lintian: homepage-field-uses-insecure-uri
See https://lintian.debian.org/tags/homepage-field-uses-insecure-uri.html for more details.

Yay consistency, and yay demanding to use proper typographic dashes instead
of minuses but on some distributions not accepting them as Unicode.

Closes #19

In Debian they got split out of btrfs-progs into libbtrfs-dev.

Even though we're root, can still happen on sshfs or such mounts.

Doubled / is harmless (not on Windows...), but would confuse people.

The latter should be easy to fix if the radix implementation is replaced.

Signed-off-by: Timofey Titovets <nefelim4ag@gmail.com>