(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/Sector-ownership.t

@@ -0,0 +1,31 @@
+.TH Concept "Sector ownership"
+.NA Sector-ownership "Sector ownership"
+.LV Basic
+.s1
+In order to own a sector, you must have at least one of the following
+present in the sector:
+
+.NF
+1) mil
+2) a land unit
+3) non-captured civilians
+.FI
+
+.s1
+If at any time you do not have at least one of those 3 things in a sector,
+you will lose control of it.
+If you are not the old owner at the time you lose the sector,
+it will revert to the old owner.  Otherwise it will
+revert to the deity, country #0, even if another country's land units
+are present in the sector.
+If there are no people at all in the sector, it will revert to the deity.
+.s1
+Note that this means that if you attack a sector and do not move in,
+you will not gain possession of it. If there are civilians in the sector,
+the sector will revert back to the original owner. If there are no civilians
+in the sector (and no mil or units, because you just killed them), the sector
+will revert back to the deity.
+.s1
+Similarly, if you abandon a sector, it'll revert immediately.
+.s1
+.SA "attack, Sectors, Occupation"