(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/Bridges.t

@@ -0,0 +1,34 @@
+.TH Concept "Bridges"
+.NA Bridges "Building and destroying bridges"
+.LV Expert
+.s1
+BRIDGES
+.s1
+Normally bridges can only be built from bridge-heads. (See info
+\*QSector-types\*U). If the EASY_BRIDGES option is in use,
+bridges can be built from any sector
+that has enough hcm (heavy construction materials) and
+workforce (see \*Qinfo show\*U and the \*Qshow\*U command),
+providing you have enough money, and your technology level is high enough.
+.s1
+Bridges can only be built over water sectors.
+.s1
+If the EASY_BRIDGES option is in use,
+you do not need to use bridgehead sectors (although the sector you
+build from must still be at least 60% efficient), and
+the sector the bridge span is to be built in must be adjacent to
+at least 1 non-sea, non-bridge span sector. Note that you do not have to
+OWN the sector is will be adjacent to, it just has to exist. This means that
+you can build, for example, a 2 sector bridge out to an unowned island.
+.s1
+To find out the required amounts of materials, workforce, and money
+use the show command.
+.s1
+Bridge spans cannot be re-designated.
+A bridge span will collapse when it is below 20% in efficiency,
+so shelling will destroy one, as will nuclear weapons and bombing.
+.s1
+If the EASY_BRIDGES option is NOT in use, a bridge will also collapse
+if its bridge-head sector goes below 20% or is re-designated.
+.s1
+.SA "build, Technology, show, Sectors"