texlive: 2017 -> 2017-final

This commit rebuilds texlive 2017 with the final release of 2017. As described
in these issues [1][2][3], the upstream CTAN mirrors are a continuously moving
rolling release without historical archives.

This particular FTP server is also a rolling release folling CTAN for the latest
version, but it has snapshots of the final texlive releases; it appears that the
2017 distribution has been unmodified since texlive-2018 was released earlier
this year.

Along the way, we needed to fix several issues:
- xindy: if $HOME is unset, it will try to mkdir /homeless-shelter, which fails
  due to insufficient permissions.
- scheme-infraonly: this scheme had symlinks into other releases that were
  read-only, so it couldn't patch and modify the scripts. This commit removes it
  for now, but that's not a particularly satisfying solution. Ideas?

This also adds some documentation on the upgrade process to prepare for
texlive-2018 [4].

This commit also replaces the sha1 hashes with upstream's standard sha512 hashes.
It appears the motivation for the shorter hashes was to save disk space in the
derivations; in master, the size of this directory is 1012K; in this commit it
is 1600K. The difference is not particularly large, and the downsides to using
our own sha1 hashes are:

- More nix code to maintain
- Multi-step upgrade process for maintainers: the maintainer first has to
  download all upstream tarballs by sha512 hash, then run the fix script, then
  rebuild with sha1 hashes.
- Less transparent. If we use the upstream sha512 hashes, any user can
  immediately verify that the hashes we're providing match upstream, or match
  the snapshot in time.
- Easier to debug. Since upstream is rolling and packages may disappear or fail
  to build, it's useful to be able to determine if the sha mismatch is because
  of an update or not; if we have a sha1 mismatch and no tarball to pull, we
  can't figure out which sha512sum would have produced that sha1.
- Less trust required. Due to the above, users don't have to trust the
  content-addressed mirrors on IPFS and @veprbl's servers as much.
- Easier to cobble together a source distribution from a variety of sources. It
  seems some FTP servers have more/less than others, or older/newer packages. If
  we know what we're looking for beforehand and we're just missing a few
  packages whose hashes match the advertised hashes upstream, it's easier to find.

[1] https://github.com/NixOS/nixpkgs/issues/24683
[2] https://github.com/NixOS/nixpkgs/issues/10026
[3] https://github.com/NixOS/nixpkgs/issues/34490
[4] https://github.com/NixOS/nixpkgs/issues/40232