Automatically retry to download packages again later if failed (by Yusuke Terada)

I often encounter the following error during network installation of TeX Live.

```
TLPDB::_install_package: downloading did not succeed
Installation failed.
```
This occurs irregularly, that is, at which package the installation aborts
varies every time. So, it is probably due to some random network troubles.

Once the error occurs, the entire installation aborts there, and users should
retry from the beginning. So I have tried implementing a workaround in the
installation routine based on Hironobu Yamashita's suggestion. His idea is:

If download of some packages fails, skip them and store them in @packs_again.
After the installation of other packages completed, retry to install the
packages stored in @packs_again.
If it fails again, the installation process is to be aborted this time around.
However, if the errors are due to random network troubles, the probability of
an error occurring twice on the same package should be quite small.
As an experiment, I tried raising network troubles intentionally during network
installation. This workaround seems to work well.

git-svn-id: svn://tug.org/texlive/trunk@53343 c570f23f-e606-0410-a88d-b1316a301751

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