texinfo.tex: Update to upstream version

Relevant fixes for LilyPond's documentation:

• improve handling of @need
• fix a vertical spacing bug of @multitable environments with a @headitem line
• correct positioning of @cartouche at the top of a page, also fixing its right margin
• the table of contents stays within the textblock area
• fix jumping horizontal space width between 'Chapter N:' and the following text in headers
• improve vertical positioning of @image objects; this makes the NR shorter by approx. 20 pages (with a corresponding adjustment to common-macros.texi for inline images)
• fix vertical spacing of @center @image
• added support for 'microtypography', similar to the 'microtype' package for LaTeX – since we use XeTeX, we unfortunately only get character protrusion and no font expansion (IMHO, this latter feature alone would be a very good reason to switch to luatex)
• the internal command @ifmonospace was replaced with @ifusingtt, making it necessary to update cyrillic.itexi accordingly

This commit also sets the txicodevaristt flag to undo a planned new behaviour of @var that would clash heavily with our use of this function.