Implement fstatat

Linux's variant uses the syscall as intended. Redox's variant uses fpath
to build a path to pass to fstat from the file descriptor plus the file
name. Unlike the syscall, this isn't atomic so the liminal space between
fpath/getcwd and fstat is subject to TOCTOU.

Beyond fstatat, I moved the stat test to its correct location and added
an assert since the output of the test is unchecked.

I also added AT_FDCWD which seems to be -100 across Unixes. The other
AT_* constants are unimplemented for now.

src/platform/linux/mod.rs

@@ -7,6 +7,7 @@ use crate::{
     header::{
         dirent::dirent,
         errno::EINVAL,
+        fcntl::{AT_EMPTY_PATH, AT_FDCWD, AT_REMOVEDIR, AT_SYMLINK_NOFOLLOW},
         signal::SIGCHLD,
         sys_resource::{rlimit, rusage},
         sys_stat::{stat, S_IFIFO},
@@ -26,10 +27,6 @@ mod ptrace;
 mod signal;
 mod socket;
 
-const AT_FDCWD: c_int = -100;
-const AT_EMPTY_PATH: c_int = 0x1000;
-const AT_REMOVEDIR: c_int = 0x200;
-
 const SYS_CLONE: usize = 56;
 const CLONE_VM: usize = 0x0100;
 const CLONE_FS: usize = 0x0200;
@@ -185,6 +182,15 @@ impl Pal for Sys {
         e_raw(unsafe { syscall!(NEWFSTATAT, fildes, empty_ptr, buf, AT_EMPTY_PATH) }).map(|_| ())
     }
 
+    unsafe fn fstatat(
+        fildes: c_int,
+        path: *const c_char,
+        buf: *mut stat,
+        flags: c_int,
+    ) -> Result<()> {
+        e_raw(unsafe { syscall!(NEWFSTATAT, fildes, path, buf, flags) }).map(|_| ())
+    }
+
     unsafe fn fstatvfs(fildes: c_int, buf: *mut statvfs) -> Result<()> {
         let mut kbuf = linux_statfs::default();
         let kbuf_ptr = &mut kbuf as *mut linux_statfs;