libstd: Add thread unsafety warnings around setenv() and unsetenv()

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 (and the answer appears to
be no).

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

Author: cgwalters

src/libstd/env.rs

@@ -241,7 +241,10 @@ impl Error for VarError {
 }
 
 /// Sets the environment variable `k` to the value `v` for the currently running
-/// process.
+/// process.  Warning: On some operating systems, this function manipulates global
+/// state in a thread-unsafe fashion.  On most Unix implementations, it will be
+/// safe to call this only very early in process startup, before any other threads
+/// are created that will potentially call `getenv()`.
 ///
 /// # Examples
 ///
@@ -261,6 +264,10 @@ 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: On some operating systems, this function manipulates global
+/// state in a thread-unsafe fashion.  On most Unix implementations, it will be
+/// safe to call this only very early in process startup, before any other threads
+/// are created that will potentially call `getenv()`.
 ///
 /// # Examples
 ///