Making it possible to use INFOPATH with Emacs info files

[Reuben Thomas]

Because I sometimes install programs in my home directory, I use INFOPATH to 
specify the system directories plus one in my home directory. However, this 
gets me into trouble with Emacs, because it takes my INFOPATH for its 
default info directory list:

  This variable `Info-default-directory-list' is used as the default
  for initializing `Info-directory-list' when Info is started, unless
  the environment variable INFOPATH is set.

which means that if I set INFOPATH, then I don't get access to Emacs's own 
info files (under /usr/share/info/emacs-22) unless I manually add this path 
to Info-directory-list.

With info, this works, because info searches subdirectories of INFOPATH 
directories for files. Emacs seems not to.

It seems to me that Emacs's info reader should always behave like that of 
the standalone info reader. Hence, Emacs should be made to look in 
subdirectories of Info-directory-list for info files.

(There's a separate issue that with the present semantics of INFOPATH it's 
impossible for a user simply to extend the system path. I've written to 
bug-texinfo@gnu.org about this. One solution would be to allow an empty path 
segment to mean "insert system path here" as it does with some other path 
variables. But any such solution should be adopted by the standalone info 
reader first, I think, to give Emacs a behaviour to follow, rather than the 
other way around.)

(This bug came up in Debian bug

http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=446376

)

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