(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/trade.t

@@ -0,0 +1,35 @@
+.TH Command TRADE
+.NA trade "Buy planes/ships/land units"
+.LV Expert
+.SY "trade"
+The trade report lists
+all ships, planes and land units that are being sold and allows you
+to buy them.
+.s1
+It is based on a bidding system, and after the bidding time has passed,
+the transactions are executed, and the highest bidder gets the goods
+(maybe.)  If there are no bidders once the time passes, the goods stay
+on the market and the first bidder will get them.
+.s1
+Your bid must be higher than the currently high bid.
+.s1
+Typical output looks like:
+.EX trade
+.NF
+
+     Empire Trade Report
+  Wed Mar 13 14:42:45 1996
+ lot high bid  by time left owner  description
+ --- --------  -- --------- -----  -------------------------
+   0 $   1000   2  0.06 hrs (  2)  tech 0 100% inf1  infantry 1 [f:21 ] #3
+Which lot to buy: 0
+Destination sector: 4,0
+How much do you bid: 1100
+Your bid on lot #0 is being considered.
+.FI
+.s1
+The cargo of ships and land units are listed in the square
+brackets, in this case, 4 guns and 40 shells.  The nuclear armament of
+planes is listed in parentheses.
+.s1
+.SA "set, Commerce"