platform/redox: migrate more functions to use their *at variants

Additionally, this adds the `SYMLOOP_MAX` constant in `limits.h` to
replace the `MAX_LEVEL` magic number used during symlink resolution.

`fstat` was kept because it is more difficult than the others to do a
drop-in replacement in terms of `fstatat` for since a naive approach
would cause mutual recursion.

src/header/limits/mod.rs

@@ -129,3 +129,4 @@ pub const PTHREAD_DESTRUCTOR_ITERATIONS: c_long = _POSIX_THREAD_DESTRUCTOR_ITERA
 // TODO: What should this limit be? Both glibc and musl have it as 1024
 pub const PTHREAD_KEYS_MAX: c_long = 4096 * 32;
 pub const PTHREAD_STACK_MIN: c_long = 65536;
+pub const SYMLOOP_MAX: c_long = 64;