libstd: Add non-Rust threadsafety warnings for setenv()

See:
https://sourceware.org/bugzilla/show_bug.cgi?id=4887#c9
https://bugs.freedesktop.org/show_bug.cgi?id=65681

I just noticed this while talking to someone who was using
`os.environ['FOO'] = 'BAR'` in Python and since I'm learning Rust, I
was curious if it did anything special here.  It looks like Rust has
an internal mutex, which helps for apps that are pure Rust, but it
will be an evil trap for someone later adding in native code (apps
like Servo and games will be at risk).

Java got this right by disallowing `setenv()` from the start.

Author: cgwalters

src/libstd/env.rs

@@ -243,6 +243,13 @@ impl Error for VarError {
 /// Sets the environment variable `k` to the value `v` for the currently running
 /// process.
 ///
+/// Warning: While an an internal mutex is used to ensure concurrent
+/// access is safe in Rust code calling `libstd`, there is no such
+/// protection for non-Rust threads the same process concurrently
+/// invoking the underlying system `getenv()` (on Unix at least).
+/// Avoid calling this API except very early in process startup,
+/// when it is known that no other threads are running.
+///
 /// # Examples
 ///
 /// ```
@@ -262,6 +269,13 @@ pub fn set_var<K: ?Sized, V: ?Sized>(k: &K, v: &V)
 
 /// Removes an environment variable from the environment of the currently running process.
 ///
+/// Warning: While an an internal mutex is used to ensure concurrent
+/// access is safe in Rust code calling `libstd`, there is no such
+/// protection for non-Rust threads the same process concurrently
+/// invoking the underlying system `getenv()` (on Unix at least).
+/// Avoid calling this API except very early in process startup,
+/// when it is known that no other threads are running.
+///
 /// # Examples
 ///
 /// ```