[xml] Runtime parser limit for maximum size of text nodes

[Nick Wellnhofer <wellnhofer aevum de>]

Greetings,

I'd like to add a runtime parser setting for the maximum size of text nodes. 
This relieves many users from setting the unsafe XML_PARSE_HUGE option. See 
for example:

    https://bugzilla.gnome.org/show_bug.cgi?id=719350
    https://bugzilla.gnome.org/show_bug.cgi?id=771799

Here's the commit:

    https://github.com/nwellnhof/libxml2/commit/7343130

I keep it up for review for a while, then push to Git master unless I hear 
objections.

Nick