Move demangling-related code from common.h to demangle.h

Adjust users accordingly.
diff --git a/options.c b/options.c
index 693676b..93dea31 100644
--- a/options.c
+++ b/options.c
@@ -39,6 +39,7 @@
 #include "common.h"
 #include "filter.h"
 #include "glob.h"
+#include "demangle.h"
 
 #ifndef SYSCONFDIR
 #define SYSCONFDIR "/etc"