Allow openat, subject to the same restrictions as open.

[khuey]

glibc 2.26 switches from using open(2) to openat(2). And now everyone on earth with a seccomp filter gets to update it :)

[jdm]

error[E0425]: cannot find value `O_DIRECTORY` in this scope
   --> platform/linux/seccomp.rs:272:55
    |
272 |                 filter.if_arg2_hasnt_set(!(O_RDONLY | O_DIRECTORY | O_CLOEXEC | O_NOCTTY | O_NONBLOCK | O_NOFOLLOW) as u32,
    |                                                       ^^^^^^^^^^^ not found in this scope
    |
help: possible candidate is found in another module, you can import it into scope
    |
19  | use libc::O_DIRECTORY;
    |
error[E0425]: cannot find value `O_NOFOLLOW` in this scope
   --> platform/linux/seccomp.rs:272:105
    |
272 |                 filter.if_arg2_hasnt_set(!(O_RDONLY | O_DIRECTORY | O_CLOEXEC | O_NOCTTY | O_NONBLOCK | O_NOFOLLOW) as u32,
    |                                                                                                         ^^^^^^^^^^ not found in this scope
    |
help: possible candidate is found in another module, you can import it into scope
    |
19  | use libc::O_NOFOLLOW;
    |
warning: unused `#[macro_use]` import
  --> lib.rs:11:1
   |
11 | #[macro_use]
   | ^^^^^^^^^^^^
   |
   = note: #[warn(unused_imports)] on by default
error[E0308]: mismatched types
   --> platform/linux/seccomp.rs:271:34
    |
271 |             filter.if_syscall_is(libc::SYS_openat, |filter| {
    |                                  ^^^^^^^^^^^^^^^^ expected u32, found i64
    |
    = help: here are some functions which might fulfill your needs:
            - .count_ones()
            - .count_zeros()
            - .leading_zeros()
            - .trailing_zeros()
error[E0599]: no method named `if_arg2_hasnt_set` found for type `&mut platform::linux::seccomp::Filter` in the current scope
   --> platform/linux/seccomp.rs:272:24
    |
272 |                 filter.if_arg2_hasnt_set(!(O_RDONLY | O_DIRECTORY | O_CLOEXEC | O_NOCTTY | O_NONBLOCK | O_NOFOLLOW) as u32,
    |                        ^^^^^^^^^^^^^^^^^
error: aborting due to 4 previous errors
error: Could not compile `gaol`.