Add slashguard feature

New command `/slashguard` tries to protect against typing ` /quit` by
not allowing a slash in the first 4 characters.

src/config/preferences.c

@@ -1744,6 +1744,7 @@ _get_group(preference_t pref)
         case PREF_STATUSBAR_ROOM:
         case PREF_TITLEBAR_MUC_TITLE:
         case PREF_HISTORY_COLOR_MUC:
+        case PREF_SLASH_GUARD:
             return PREF_GROUP_UI;
         case PREF_STATES:
         case PREF_OUTTYPE:
@@ -2051,6 +2052,8 @@ _get_key(preference_t pref)
             return "history.muc.color";
         case PREF_AVATAR_CMD:
             return "avatar.cmd";
+        case PREF_SLASH_GUARD:
+            return "slashguard";
         default:
             return NULL;
     }

src/config/preferences.h

@@ -167,6 +167,7 @@ typedef enum {
     PREF_CORRECTION_ALLOW,
     PREF_HISTORY_COLOR_MUC,
     PREF_AVATAR_CMD,
+    PREF_SLASH_GUARD,
 } preference_t;
 
 typedef struct prof_alias_t {