fixup! X wcfs: client: Handle fork

Starting from 3f83469c Conn and WatchLink started to inherit from
interface, which made them to use virtual functions, which, without
destructor being also virtual emits the following warnings:

    wcfs/client/wcfs.cpp: In member function ‘virtual void wcfs::_Conn::decref()’:
    wcfs/client/wcfs.cpp:1531:16: warning: deleting object of polymorphic class type ‘wcfs::_Conn’ which has non-virtual destructor might cause undefined behavior [-Wdelete-non-virtual-dtor]
             delete this;
                    ^~~~

    wcfs/client/wcfs_watchlink.cpp: In member function ‘virtual void wcfs::_WatchLink::decref()’:
    wcfs/client/wcfs_watchlink.cpp:514:16: warning: deleting object of polymorphic class type ‘wcfs::_WatchLink’ which has non-virtual destructor might cause undefined behavior [-Wdelete-non-virtual-dtor]
             delete this;
                    ^~~~

wcfs/client/wcfs.h

@@ -190,7 +190,7 @@ struct _Conn : os::_IAfterFork, object {
     // don't new - create via WCFS.connect
 private:
     _Conn();
-    ~_Conn();
+    virtual ~_Conn();
     friend pair<Conn, error> WCFS::connect(zodb::Tid at);
 public:
     void incref();

wcfs/client/wcfs_watchlink.h

@@ -99,7 +99,7 @@ public:
     // don't new - create only via WCFS._openwatch()
 private:
     _WatchLink();
-    ~_WatchLink();
+    virtual ~_WatchLink();
     friend pair<WatchLink, error> WCFS::_openwatch();
 public:
     void incref();