(all): Depend on info.
Flatten info directory. This undoes the move to one subdirectory per
chapter, which was done during Empire 2. The structure doesn't buy us
much, as the info name space is flat, and it complicates makefiles.
Overhaul info.pl:
- It now wants to run in the root of the build tree.
- Information on source files and subjects is now stored in makefiles,
thus info.pl no longer picks up random junk from the file system.
- Clean up Perl anachronisms, in particular use subroutine arguments and
results rather than global variables where convenient.
- Change format of diagnostics to the common format used by GNU tools,
so that Emacs and the like can parse it.
- Catch missing .SA.
- When creating a new subject file, cowardly refuse to overwrite an
existing file.
- Subject files contain topics sorted by chapter, then by name. The
order of chapters used to depend on how Perl sorts hash keys. Fix
it.
