(info, html): Implement.

(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.

info/bye.t

@@ -0,0 +1,15 @@
+.TH Command BYE
+.NA bye "Log out of Empire"
+.LV Basic
+.SY "bye"
+The bye command is how you go \*Qbye-bye\*U to log out of the game.
+.s1
+This command costs zero BTUs.  You can use it often.
+.s1
+If, for some obscure reason,
+you would like to end an Empire session
+but prefer not to use the \*Qbye\*U command
+you may achieve the same result by typing the
+EOT character, (control-D).
+.s1
+.SA "Playing"