Implement rand_r(), strnlen_s(), tempnam(), tmpnam()

The RNG for rand_r() is pretty weak, but if you're using rand_r() you probably aren't looking for robust RNG anyway.

tempnam() and tmpnam() just use mktemp() to generate file names.

src/header/stdlib/mod.rs

 //! stdlib implementation for Redox, following http://pubs.opengroup.org/onlinepubs/7908799/xsh/stdlib.h.html
 
 use core::{intrinsics, iter, mem, ptr, slice};
-use rand::distributions::Alphanumeric;
+use rand::distributions::{Alphanumeric, Distribution, Uniform};
 use rand::prng::XorShiftRng;
 use rand::rngs::JitterRng;
 use rand::{Rng, SeedableRng};
@@ -34,6 +34,10 @@ pub const MB_LEN_MAX: c_int = 4;
 static mut ATEXIT_FUNCS: [Option<extern "C" fn()>; 32] = [None; 32];
 static mut RNG: Option<XorShiftRng> = None;
 
+lazy_static! {
+    static ref RNG_SAMPLER: Uniform<c_int> = Uniform::new_inclusive(0, RAND_MAX);
+}
+
 #[no_mangle]
 pub extern "C" fn _Exit(status: c_int) {
     unistd::_exit(status);
@@ -618,10 +622,10 @@ pub extern "C" fn qsort(
 #[no_mangle]
 pub unsafe extern "C" fn rand() -> c_int {
     match RNG {
-        Some(ref mut rng) => rng.gen_range(0, RAND_MAX),
+        Some(ref mut rng) => RNG_SAMPLER.sample(rng),
         None => {
             let mut rng = XorShiftRng::from_seed([1; 16]);
-            let ret = rng.gen_range(0, RAND_MAX);
+            let ret = RNG_SAMPLER.sample(&mut rng);
             RNG = Some(rng);
             ret
         }
@@ -637,7 +641,7 @@ pub unsafe extern "C" fn rand_r(seed: *mut c_uint) -> c_int {
         let seed_arr: [u8; 16] = mem::transmute([*seed; 16 / mem::size_of::<c_uint>()]);
 
         let mut rng = XorShiftRng::from_seed(seed_arr);
-        let ret = rng.gen_range(0, RAND_MAX);
+        let ret = RNG_SAMPLER.sample(&mut rng);
 
         *seed = ret as _;