Used a shared uv_mutex_t in unix implementations of these functions
to prevent simultaneous execution.
Fixes: https://github.com/libuv/libuv/issues/271
PR-URL: https://github.com/libuv/libuv/pull/1640
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
