Relax restrictions on unknown feature directives

Instead of forcibly always aborting compilation, allow usage of
 #[warn(unknown_features)] and related lint attributes to selectively abort
 compilation. By default, this lint is deny.

src/test/compile-fail/gated-bad-feature.rs

@@ -13,9 +13,8 @@
     foo(bar),
     foo = "baz"
 )];
-//~^^^^ ERROR: unknown feature
-//~^^^^ ERROR: malformed feature
-//~^^^^ ERROR: malformed feature
+//~^^^ ERROR: malformed feature
+//~^^^ ERROR: malformed feature
 
 #[feature]; //~ ERROR: malformed feature
 #[feature = "foo"]; //~ ERROR: malformed feature

src/test/compile-fail/lint-unknown-feature.rs

@@ -0,0 +1,15 @@
+// Copyright 2013 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#[deny(unknown_features)];
+
+#[feature(this_is_not_a_feature)]; //~ ERROR: unknown feature
+
+fn main() {}