Add slashguard feature

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

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;