(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/Maintenance.t

@@ -0,0 +1,31 @@
+.TH Concept "Maintenance"
+.NA Maintenance "The cost to maintain ships, planes, and land units."
+.LV Expert
+During each update, nations must pay to maintain their units, ships and planes.
+.s1
+The cost to maintain a ship, or land-unit is:
+.NF
+	.001 * cost of thing * ETUs/update
+.FI
+.s1
+The cost to maintain a plane is:
+.NF
+	.001 * cost of thing * ETUs/update + 5 * mil_cost * crew
+.FI
+.s1
+Note that this cost can be changed by the deity.
+.s1
+Also note that if you have purchased high-tech items, and your tech
+is not at least 85% of the item's tech, you are charged double
+to maintain the item.
+.s1
+Also note that engineering land units costs 3 times what
+normal units cost, after the tech is figured in.
+.s1
+If you can not afford to pay maintenance costs or you choose not to
+pay them (see info budget) then your unit will lose ETU/5 points of
+efficiency.  Thus, if you were playing in a 60 ETU game, and you did
+not pay your maintenance costs for a 50% frigate, then the frigate
+would drop to 38% efficiency.
+.s1
+.SA "Unit-types, Ship-types, Plane-types, Ships, LandUnits, Planes, Updates"