Use pref_t rather than struct preference.

Consistently use it (except in some of the C++ code, where, for some
unknown reason, some headers, such as epan/prefs.h, are not being
included).

While we're at it, don't call prefs_find_module("extcap") twice, just
call it once and save the result.

epan/prefs.c

@@ -1152,7 +1152,7 @@ module_find_pref_cb(const void *key _U_, void *value, void *data)
 
 /* Tries to find a preference, setting containing_module to the (sub)module
  * holding this preference. */
-static struct preference *
+static pref_t *
 prefs_find_preference_with_submodule(module_t *module, const char *name,
         module_t **containing_module)
 {
@@ -1184,10 +1184,10 @@ prefs_find_preference_with_submodule(module_t *module, const char *name,
     if (containing_module)
         *containing_module = arg.submodule ? arg.submodule : module;
 
-    return (struct preference *) list_entry->data;
+    return (pref_t *) list_entry->data;
 }
 
-struct preference *
+pref_t *
 prefs_find_preference(module_t *module, const char *name)
 {
     return prefs_find_preference_with_submodule(module, name, NULL);

extcap.c

@@ -829,25 +829,26 @@ void extcap_free_if_configuration(GList *list, bool free_args)
     g_list_free(list);
 }
 
-struct preference *
+pref_t *
 extcap_pref_for_argument(const char *ifname, struct _extcap_arg *arg)
 {
-    struct preference *pref = NULL;
+    pref_t *pref = NULL;
 
     extcap_ensure_all_interfaces_loaded();
 
     GRegex *regex_name = g_regex_new("[-]+", G_REGEX_RAW, (GRegexMatchFlags) 0, NULL);
     GRegex *regex_ifname = g_regex_new("(?![a-zA-Z0-9_]).", G_REGEX_RAW, (GRegexMatchFlags) 0, NULL);
     if (regex_name && regex_ifname)
     {
-        if (prefs_find_module("extcap"))
+        module_t *extcap_module = prefs_find_module("extcap");
+        if (extcap_module)
         {
             char *pref_name = g_regex_replace(regex_name, arg->call, strlen(arg->call), 0, "", (GRegexMatchFlags) 0, NULL);
             char *ifname_underscore = g_regex_replace(regex_ifname, ifname, strlen(ifname), 0, "_", (GRegexMatchFlags) 0, NULL);
             char *ifname_lowercase = g_ascii_strdown(ifname_underscore, -1);
             char *pref_ifname = g_strconcat(ifname_lowercase, ".", pref_name, NULL);
 
-            pref = prefs_find_preference(prefs_find_module("extcap"), pref_ifname);
+            pref = prefs_find_preference(extcap_module, pref_ifname);
 
             g_free(pref_name);
             g_free(ifname_underscore);

extcap.h

@@ -248,7 +248,7 @@ extcap_request_stop(capture_session *cap_session);
  * @param arg The command line argument to check.
  * @return The associated preference on success, NULL on failure.
  */
-struct preference *
+pref_t *
 extcap_pref_for_argument(const char *ifname, struct _extcap_arg * arg);
 
 /**

ui/qt/extcap_argument.cpp

@@ -936,7 +936,7 @@ int ExtcapArgument::argNr() const
 
 QString ExtcapArgument::prefKey(const QString & device_name)
 {
-    struct preference * pref = NULL;
+    pref_t * pref = NULL;
 
     if (_argument == 0 || ! _argument->save)
         return QString();

ui/qt/packet_dialog.h

@@ -38,7 +38,7 @@ class PacketDialog : public WiresharkDialog
 
 signals:
     void showProtocolPreferences(const QString module_name);
-    void editProtocolPreference(struct preference *pref, struct pref_module *module);
+    void editProtocolPreference(pref_t *pref, struct pref_module *module);
 
 private slots:
     void on_buttonBox_helpRequested();

ui/qt/packet_list.h

@@ -179,7 +179,7 @@ protected slots:
     void editColumn(int column);
     void packetListScrolled(bool at_end);
     void showProtocolPreferences(const QString module_name);
-    void editProtocolPreference(struct preference *pref, struct pref_module *module);
+    void editProtocolPreference(pref_t *pref, struct pref_module *module);
 
     void framesSelected(QList<int>);
     void fieldSelected(FieldInformation *);

ui/qt/proto_tree.h

@@ -76,7 +76,7 @@ class ProtoTree : public QTreeView
     void goToPacket(int);
     void relatedFrame(int, ft_framenum_type_t);
     void showProtocolPreferences(const QString module_name);
-    void editProtocolPreference(struct preference *pref, struct pref_module *module);
+    void editProtocolPreference(pref_t *pref, struct pref_module *module);
 
 public slots: