remove compile-time Linux version check, replace with feature

The Linux version check was overly restrictive, even on systems that did have the right kernel
version installed but had older headers in `/usr/include/linux`. Beyond that, this check made it
more difficult to compile on a different host than what's targeted.

There is now a `linux4_14` feature flag on `userfaultfd-sys`, which turns on and tests the extra
constants available in that version. Since `userfaultfd` did not make use of any of those newer
features, it doesn't have a feature flag yet.

Applications should take care when initializing with `UffdBuilder` to specify the features and
ioctls they require, so that an unsupported version will be detected at runtime.

Bonus: disable default-features on the `bindgen` dependency, so we don't have to build `clap` etc.

Author: acfoltzer

.gitignore

@@ -5,3 +5,4 @@ TAGS
 package
 *.pyc
 core.*
+Cargo.lock