Add slashguard feature

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

src/ui/console.c

@@ -2049,6 +2049,16 @@ cons_avatar_setting(void)
     prefs_free_string(pref);
 }
 
+void
+cons_slashguard_setting(void)
+{
+    if (prefs_get_boolean(PREF_SLASH_GUARD)) {
+        cons_show("Slashguard (/slashguard)       : ON");
+    } else {
+        cons_show("Slashguard (/slashguard)       : OFF");
+    }
+}
+
 void
 cons_show_connection_prefs(void)
 {

src/ui/inputwin.c

@@ -196,6 +196,13 @@ inp_readline(void)
     }
 
     if (inp_line) {
+        if (!get_password && prefs_get_boolean(PREF_SLASH_GUARD)) {
+            char *res = (char*) memchr (inp_line+1, '/', 3);
+            if (res) {
+                cons_show("Your text contains a slash in the first 4 characters");
+                return NULL;
+            }
+        }
         return strdup(inp_line);
     } else {
         return NULL;

src/ui/ui.h

@@ -320,6 +320,7 @@ void cons_color_setting(void);
 void cons_os_setting(void);
 void cons_correction_setting(void);
 void cons_avatar_setting(void);
+void cons_slashguard_setting(void);
 void cons_show_contact_online(PContact contact, Resource *resource, GDateTime *last_activity);
 void cons_show_contact_offline(PContact contact, char *resource, char *status);
 void cons_theme_properties(void);