Wrong mathsurround support

[jlaurens]

The support for \mathsurround is not accurate: The math node is always located before the horizontal space, which is wrong for opening math nodes.

Things must be changed in tex engines (pdftex, xetex and luatex), I am on it and will push some updates to texlive once this is done, except for luatex.

This is also related to issue #30