[tex-live] Add user directory

[Karl Berry]

Hi Igor,

I appreciate the suggestion for more documentation on how to do
this, but it's not clear to me where or how anything should be added.
TEXMFHOME is discussed in the main manual (texlive-en.tex, etc.), at least.

The easiest is to set TEXMFHOME during the installation.

If already installed, I believe
  tlmgr conf texmf TEXMFHOME "d:\local"
should do it (though Windows quoting and backslashes is not something I
know about). This command edits the .../texlive/2017/texmf.cnf file.
You can do that too, as described in the comments at the top of that file.

BTW, inspired by MiKTeX's additional trees feature, we also made a tlmgr
option to manage auxiliary trees, namely "conf auxtrees":

http://tug.org/texlive/doc/tlmgr.html#conf-auxtrees---conffile-file-show-add-delete-value

(Sorry for the long url.)

A final alternative is to set the TEXMFHOME envvar, but I don't
recommend it, just because setting envvars here and there is one of the
most common sources of problems.

Hope something in there helps,
Karl