Merge from vendor branch TNF:

Merge from vendor branch TNF:
Import tex-underscore-2010 as print/tex-underscore.

With the package, \_ in text mode (i.e., \textunderscore) prints an
underscore so that hyphenation of words either side of it is not
affected; a package option controls whether an actual hyphenation point
appears after the underscore, or merely a break point. The package also
arranges that, while in text, '_' itself behaves as \textunderscore (the
behaviour of _ in maths mode is not affected.