Migrate a bunch of code out of common.h

Put it into wcstringutil, path, or a new file null_terminated_array.

src/future_feature_flags.cpp

@@ -4,6 +4,8 @@
 
 #include <cwchar>
 
+#include "wcstringutil.h"
+
 /// The set of features applying to this instance.
 static features_t global_features;