#203 pgfplots shouldn't use \write16

[U_Fischer]

luatex has more write streams and \write16 is not special with this engine. This means that the messages of pgfplots which uses \immediate\write16 can end up in other open files. I don't have an example for pgfplots but one for pgf just came up: https://sourceforge.net/p/pgf/bugs/488/ and I think it is probably that one can build a similar for pgfplots.

The suggestion of the latex team is "to be portable and use an unopened stream you can use any number bigger than 127 or use \@unused"
http://tug.org/pipermail/luatex/2015-November/005485.html