Fix #251 and some bitrot in enter_grain

This is one commit instead of two because we didn't want to bloat the
repository with two updates to the executable. We should really just
remove the binary from the repo and build it; see #253

The bitrot:

With #![feature(alloc_system)] I get an error from the rust toolchain
that the feature has been removed. We don't appear to actually be using
this anyway, so let's just remove it.

helpers/enter-grain-source/src/main.rs

@@ -1,8 +1,6 @@
-#![feature(alloc_system)]
-extern crate alloc_system;
-
 // use std::env;
-use std::fs::File;
+use std::fs::{File, metadata};
+use std::os::unix::fs::MetadataExt;
 use std::io::Read;
 use std::ptr;
 use std::env;
@@ -74,13 +72,12 @@ fn wait_all_children() {
     }
 }
 
-fn setuid_setgid_1000() {
-    // These are the right UID & GID because they are what Sandstorm
-    // uses inside the user namespace.
-    let one_thousand = 1000usize;
+fn setuid_setgid() {
+    let meta = metadata("/var").unwrap();
+
     unsafe {
-        syscall!(SETUID, one_thousand);
-        syscall!(SETGID, one_thousand);
+        syscall!(SETUID, meta.uid());
+        syscall!(SETGID, meta.gid());
     }
 }
 
@@ -173,7 +170,7 @@ fn main() {
     let fork_result = fork();
     if fork_result == 0 {
         // in the child
-        setuid_setgid_1000();
+        setuid_setgid();
         execve_bash(result);
     } else {
         // in the parent

helpers/enter_grain.sha1

@@ -1 +1 @@
-11198904136a7e8079d41ef50af613d21786b5b8  enter_grain
+3e1a80bd7ca1f61de6b3044e904947264e6f6e53  enter_grain