Log Message:
libuv: updated to 1.18.0

v1.18.0:
* aix: fix -Wmaybe-uninitialized warning
* doc: remove note about SIGWINCH on Windows
* Revert "unix,win: wait for threads to start"
* unix,win: add uv_os_getpid()
* unix: remove incorrect assertion in uv_shutdown()
* doc: fix IRC URL in CONTRIBUTING.md