texinfo.tex doesn't allow underscore in flag names for @ifset etc.

[Paul Eggert]

The texinfo documentation doesn't specify the syntax of flags like
"foo_bar"; if underscore is not allowed in flag names, it would be
helpful to document that restriction.

I ran into this problem when trying to use conditional commands in the
Autoconf manual.  Here's a short example illustrating the problem:

        \input texinfo   @c -*-texinfo-*-
        @setfilename NAME-OF-INFO-FILE
        @settitle NAME-OF-MANUAL

        @ifclear foo_bar
        text
        @end ifclear

        @bye

makeinfo works, but texi2dvi (GNU Texinfo 4.0) 0.43 with the latest
stable texinfo.tex fails with the following message.

This is TeX, Version 3.1415 (C version 6.1)
(/net/knick/home/eggert/junk/foo.texi (texinfo.tex
Loading texinfo [version 2001-07-31.06]: Basics, pdf, fonts, page headings,
tables, conditionals, indexing, sectioning, toc, environments, defuns, macros,
cross references, (/opt/reb/lib/texmf/tex/plain/dvips/epsf.tex) localization,
and turning on texinfo input format.)
! Missing @endcsname inserted.
<to be read again> 
                   @unhbox 
@leavevmode ->@unhbox 
                      @address@hidden 
@_->@leavevmode 
                @kern .06em @vbox address@hidden width.3em height.1ex}
@ifusingtt ...im @fontdimen address@hidden =0pt address@hidden #2
                                                  @fi 
<argument> foo_
               bar
@next #1->@expandafter @ifx @csname SET#1
                                         @endcsname @relax @expandafter @ifc...
l.5 @ifclear foo_bar
                    
? ^D
! Emergency stop.
<to be read again> 
                   @unhbox 
@leavevmode ->@unhbox 
                      @address@hidden 
@_->@leavevmode 
                @kern .06em @vbox address@hidden width.3em height.1ex}
@ifusingtt ...im @fontdimen address@hidden =0pt address@hidden #2
                                                  @fi 
<argument> foo_
               bar
@next #1->@expandafter @ifx @csname SET#1
                                         @endcsname @relax @expandafter @ifc...
l.5 @ifclear foo_bar
                    
No pages of output.
Transcript written on foo.log.
/opt/reb/bin/texi2dvi: tex exited with bad status, quitting.
/opt/reb/bin/texi2dvi: see foo.log for errors.