info.el dots in node names

[Kevin Ryde]

[This started on the autoconf-patches list about dots in node names in
the autoconf manual, eg. "config.status".]

        * info.el (Info-following-node-name): Accept "." followed by allowed
        non-whitespace non-")" chars, so filenames like foo.h can be in node
        names.

The standalone info reader uses a similar rule (skip_node_characters
in info/search.c), though currently it includes the "." in "node.,",
which I'm not sure it should, in case someone puts a comma after an
@pxref (foo.texi below).

foo.texi below is a little sample I was testing with.  Notice with
this change that the cross references in the "something.txt" node
work, as does the index entry.  The top-level menu entry already
worked.

info.el.node-name-dots.diff
Description: Text document

foo.texi
Description: TeXInfo document