Migrate a bunch of code out of common.h

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

src/complete.cpp

@@ -47,6 +47,7 @@
 #include "reader.h"
 #include "tnode.h"
 #include "util.h"
+#include "wcstringutil.h"
 #include "wildcard.h"
 #include "wutil.h"  // IWYU pragma: keep