AGS Steam stub plugin

This is a replacement for the AGSteam plugin by monkey_05_06 that can be found
here: http://www.adventuregamestudio.co.uk/forums/index.php?topic=44712.0

It replicates its functionality and provides basic in-memory storage of state,
so games that require this plugin should run without the Steam client.
As of the current release, statistics and achievements are also written to
disk and loaded back on startup. The location of the state database is
$SAVEGAMEDIR$/agsteam.db, which will normally translate to
<Home Directory>/.local/share/ags/<Game Name>/agsteam.db

Note that this plugin was written primarily to allow playing Heroine's Quest
on Linux, for which no official release is available (yet?). Other platforms
and games are untested.
The Windows version of Heroine's Quest can be downloaded for free on Steam:
http://store.steampowered.com/app/283880

Please do not use the plugin to circumvent Steam DRM and play games you
haven't paid for! Every developer has invested countless hours into providing
you with entertainment, you should acknowledge that and support them by buying
their games legally.

The author has not signed an NDA with Valve Software, Inc. and has not had
access to any material concerning the Steam API. This software was originally
developed by extracting the string contents of AGSteam.dll, but all the
required information can also be obtained from the aforementioned thread on
adventuregamestudio.co.uk. No machine code was analyzed or reverse engineered.

To compile the plugin, a copy of agsplugin.h is needed, along with a C++
compiler, (GNU) make and associated tools. Please download the latest version
of agsplugin.h from github and copy it into the same directory as agsteam.cpp
and Makefile. Then run make, which should produce libagsteam.so. This is the
plugin. If you copy it into a game folder, ags should find and load it
automatically, just like it would do with AGSteam.dll on Windows.

Specific instructions for Debian and Ubuntu (untested):
$ sudo apt-get install g++ libc6-dev git make wget
$ git clone https://github.com/onitake/agsteamstub.git
$ cd agsteam
$ wget https://github.com/adventuregamestudio/ags/blob/master/Engine/plugin/agsplugin.h
$ make
$ cp libagsteam.so <your_game_directory>
$ cd <your_game_directory>
$ ags

This is free and unencumbered software released into the public domain.

Anyone is free to copy, modify, publish, use, compile, sell, or
distribute this software, either in source code form or as a compiled
binary, for any purpose, commercial or non-commercial, and by any
means.

In jurisdictions that recognize copyright laws, the author or authors
of this software dedicate any and all copyright interest in the
software to the public domain. We make this dedication for the benefit
of the public at large and to the detriment of our heirs and
successors. We intend this dedication to be an overt act of
relinquishment in perpetuity of all present and future rights to this
software under copyright law.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR
OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
OTHER DEALINGS IN THE SOFTWARE.

For more information, please refer to <http://unlicense.org/>