more resilient TL installation

[Karl Berry]

Norbert found the time to implement an oft-requested feature for
install-tl: for network installations only, if the download or
installation of a single package fails, it can now continue, instead of
aborting right away.  (Apart from the handful of core infrastructure
packages -- same as scheme-infraonly -- which are installed first.)

Then it retries once at the end, in case the error was transient.  If
failure persists, at a later time you can run tlmgr to install the
packages that had problems.

For now, you have to invoke
  install-tl --continue
to get the new behavior. We hoped there might be a few people who
might like to test in other environments than ours before we make this
default, which we hope/expect to do by the end of the week.

I guess the best way to test would be to start a full installation and
then disable the network connection. We haven't done that, though.
Instead, given a mirror of tlnet, we've tested this way:

1) in $TLNET/archive/, mv some .tar.xz files to simulate the broken
download. E.g., mv dvips* /tmp.

2) run:
  $TLNET/install-tl --continue --debug-fakenet --profile /tmp/TL.pro
where
1) --debug-fakenet makes install-tl think it is doing a network
install, for our testing purposes.

2) /tmp/TLmin.pro is the profile file below. Just saves the hassle of
setting all the directory variables interactively, since we don't want
to change any real directories. Adjust options to suit, of course.  This
installs the "minimal" scheme, just plain TeX, into /tmp/ki.

You should then observe install-tl failing to install the packages that
have been moved aside, retrying at the end and failing again, and then
outputting a message saying that you can get them by running:
  tlmgr update --all --reinstall-forcibly-removed
(in the newly-installed tree). Because by default the repo in the
newly-installed tree will be switched to CTAN, this will work
without having to move the packages back.

Sorry it's complicated, but that's life. Maybe this email will be
documentation if nothing else :). --thanks, karl.

P.S. One more complication: for format rebuilding, some dependencies are
awry, e.g., dehyph-exptl. This should be repaired tonight.

# TL.pro
selected_scheme scheme-minimal
TEXDIR /tmp/ki
TEXMFHOME /tmp/ki/user/home
TEXMFLOCAL /tmp/ki/texmf-local
TEXMFSYSCONFIG /tmp/ki/sys/config
TEXMFSYSVAR /tmp/ki/sys/var
TEXMFCONFIG /tmp/ki/user/config
TEXMFVAR /tmp/ki/user/var
option_doc 1
option_fmt 1
option_letter 1
option_src 1
option_symlinks 0