Passing or returning structs as values makes life hard for people that
work with libuv through a foreign function interface. Switch to a
pointer-based approach.
Fixes#684.
This changes the prototype of uv_run() from:
int uv_run(uv_loop_t* loop);
To:
int uv_run(uv_loop_t* loop, uv_run_mode mode);
Where `mode` is UV_RUN_DEFAULT, UV_RUN_ONCE or UV_RUN_NOWAIT.
Fixes#683.
Replace `void f()` with `void f(void)`; the former means "a function
that takes any number of arguments, including none" while the latter
is what is actually intended: a function taking no arguments.
The first form also isn't strictly conforming ANSI/ISO C.
