Move multiplayer stuff under multiplayer/ (part 1/2)

data/_main.cfg

@@ -1,6 +1,7 @@
 {themes/}
 {core/}
 {hardwired/}		# Locations under here are wired into the C++
+{multiplayer/}
 {campaigns/}
 {scenario-test.cfg}
 

data/core/_main.cfg

@@ -8,7 +8,6 @@
 {core/help.cfg}
 {core/hotkeys.cfg}
 {core/items.cfg}
-{core/multiplayer.cfg}
 {core/names.cfg}
 {core/schedules.cfg}
 {core/special-notes.cfg}

data/multiplayer/_main.cfg

@@ -0,0 +1,16 @@
+# Main purpose of this file is to ensure that macros get read in first.
+[generic_multiplayer]
+	id=user_map
+	name= _ "User Map"
+	turns=90
+
+	{DEFAULT_SCHEDULE}
+	{DEFAULT_MUSIC_PLAYLIST}
+[/generic_multiplayer]
+
+#ifdef MULTIPLAYER
+{multiplayer/scenarios/}
+{~scenarios/multiplayer/}
+#endif
+
+{multiplayer/eras.cfg}