Linux only plugin: use "die" command to kill game

Since you can't do "Ctrl-Z kill -9 %1" from the console, instead just
give the "die" command to terminate the game without saving.  Linux
only, since _exit() probably doesn't work on Windows.

Need to set cmake option BUILD_KILL_GAME to ON to compile this plugin.

plugins/CMakeLists.txt

@@ -34,3 +34,10 @@ DFHACK_PLUGIN(cleanmap cleanmap.cpp)
 DFHACK_PLUGIN(weather weather.cpp)
 DFHACK_PLUGIN(vdig vdig.cpp)
 DFHACK_PLUGIN(colonies colonies.cpp)
+
+IF(UNIX)
+    OPTION(BUILD_KILL_GAME "Build the kill gmae plugin." OFF)
+    if(BUILD_KILL_GAME)
+        DFHACK_PLUGIN(die die.cpp)
+    endif()
+endif()

plugins/die.cpp

@@ -0,0 +1,47 @@
+// Since you can't do "Ctrl-Z kill -9 %1" from the console, instead just
+// give the "die" command to terminate the game without saving.
+// Linux only, since _exit() probably doesn't work on Windows.
+//
+// Need to set cmake option BUILD_KILL_GAME to ON to compile this
+// plugin.
+
+#ifndef LINUX_BUILD
+    #error "This plugin only compiles on Linux"
+#endif
+
+#include <dfhack/Core.h>
+#include <dfhack/Export.h>
+#include <dfhack/PluginManager.h>
+#include <vector>
+#include <string>
+
+using std::vector;
+using std::string;
+using namespace DFHack;
+
+DFhackCExport command_result df_die (Core * c, vector <string> & parameters);
+
+DFhackCExport const char * plugin_name ( void )
+{
+    return "die";
+}
+
+DFhackCExport command_result plugin_init ( Core * c, std::vector <PluginCommand> &commands)
+{
+    commands.clear();
+    commands.push_back(PluginCommand("die",
+                       "Kill game without saving", df_die));
+    return CR_OK;
+}
+
+DFhackCExport command_result plugin_shutdown ( Core * c )
+{
+    return CR_OK;
+}
+
+DFhackCExport command_result df_die (Core * c, vector <string> & parameters)
+{
+    _exit(0);
+
+    return CR_OK;
+}