Switch to the upstream OpenBSD getenv/putenv/setenv implementation.

This fixes all the bugs found by the new tests.

Change-Id: Id5a5f9f39a0620208bafa053f871a044725b4795
diff --git a/libc/include/unistd.h b/libc/include/unistd.h
index d21f23d..6cb36d8 100644
--- a/libc/include/unistd.h
+++ b/libc/include/unistd.h
@@ -47,7 +47,8 @@
 #define SEEK_CUR 1
 #define SEEK_END 2
 
-extern char **environ;
+extern char** environ;
+
 extern __noreturn void _exit(int);
 
 extern pid_t  fork(void);