Only allow a single D main() function.

Fixes DMD testcase 'fail5634'.

gen/functions.cpp

@@ -522,6 +522,11 @@ void DtoDeclareFunction(FuncDeclaration* fdecl)
 
     // main
     if (fdecl->isMain()) {
+        // Detect multiple main functions, which is disallowed. DMD checks this
+        // in the glue code, so we need to do it here as well.
+        if (gIR->mainFunc) {
+            error(fdecl->loc, "only one main function allowed");
+        }
         gIR->mainFunc = func;
     }