[tex-live] tlcritical->tlnet for downloader/compressor

[Karl Berry]

If all goes well, we will push the current tlcritical to tlnet tonight.
It may take additional day(s) to reach the CTAN mirrors, as usual.

There is a bunch of new code from Norbert relating to the download and
compression programs used, with details as follows. (I'll add this text
to tug.org/texlive/tlmgr-news.html after the update goes through.)

Many thanks to several Mac people for testing.

Good luck to us all,
Karl

<p><b>tlmgr 47907 (released 4 June 2018):</b>
<li>support for curl as a download program, and new default behavior for
choosing. In this order:
  <ol>
  <li>If the environment variable <tt>TEXLIVE_DOWNLOADER</tt> is
      defined, use it; abort if the specified program doesn't work.
      Possible values: <tt>curl</tt>, <tt>wget</tt>.
  <li>If the environment variable <tt>TL_DOWNLOAD_PROGRAM</tt> is
      defined (can be any value), use it together with
      <tt>TL_DOWNLOAD_ARGS</tt>; abort if it doesn't work.
  <li>If LWP is available and working, use that (by far the most
      efficient method, as it supports persistent downloads).
  <li>If curl is available (from the system) and working, use that.
  <li>If wget is available (either from the system or TL) and
      working, use that.
  </ol>
  <p>TL still provides <tt>wget</tt> binaries for some platforms, so
  some download method should always be available.
  
<li>support for alternative compressor programs; until now, backups and
  intermediate rollback containers were always created with xz. Now:
  <ol>
  <li>If the environment variable <tt>TEXLIVE_COMPRESSOR</tt> is
      defined, use it; abort if it doesn't work. Possible values:
      <tt>lz4</tt>, <tt>gzip</tt>, <tt>xz</tt>.
  <li>If lz4 is available (either from the system or TL) and working, use that.
  <li>If gzip is available (from the system) and working, use that.
  <li>If xz is available (either from the system or TL) and working, use that.
  </ol>
  <p>lz4 and gzip are much faster in creating tlmgr's local backups.
  The unconditional use of xz for the tlnet containers is unaffected,
  to minimize download sizes.

<li><a href="https://openssh.com/">ssh</a> support: installing from a
  repository of the form <tt>user at machine:/path/to/tlnet</tt> is now
  supported; the url-like forms
  <tt>scp://user@machine/path/to/tlnet</tt> and
  <tt>ssh://user@machine/path/to/tlnet</tt> are equivalent. ssh-agent
  is strongly recommended!

<li>Not tlmgr, but close enough: our generic Windows wrapper
runscript.tlu now sets <tt>LC_CTYPE</tt> to be the system encoding, to
support files with non-ASCII characters.