osx: fix -Wextra warnings

On OS X 10.9.1, libuv does not compile cleanly with all warnings
enabled:

```
CFLAGS="-g -O2 -Wall -Wextra -Wno-unused-parameter -Werror"
CFLAGS="$CFLAGS --std=gnu89 -pedantic"
# The following is required for include/uv-dtrace.h
CFLAGS="$CFLAGS -Wno-dollar-in-identifier-extension"
./configure CFLAGS="$CFLAGS"
```

Most of the warnings are caused by function pointer casts in
`src/unix/darwin-proctitle.c` and `src/unix/fsevents.c`. In general I try
to avoid typedef-ing function prototypes, but in this case I use them
because it cleans up the code nicely.