fsevents: use shared FSEventStream

It seems that number of simultaneously opened FSEventStreams is
limited on OSX (i.e. you can have only fixed number of them on
one running system), getting past through this limit will cause
`FSEventStreamCreate` to return false and write following message
to stderr:

    (CarbonCore.framework) FSEventStreamStart: register_with_server:
    ERROR: f2d_register_rpc() => (null) (-21)

To prevent this, we must use only one shared FSEventStream with a
paths for all uv_fsevent_t handles, and then filter out events for
each handle using this paths again.

See nodejs/node-v0.x-archive#5463

include/uv-darwin.h

@@ -36,8 +36,8 @@
 
 #define UV_PLATFORM_LOOP_FIELDS                                               \
   uv_thread_t cf_thread;                                                      \
-  void* cf_cb;                                                                \
-  void* cf_loop;                                                              \
+  void* _cf_reserved;                                                         \
+  void* cf_state;                                                             \
   uv_mutex_t cf_mutex;                                                        \
   uv_sem_t cf_sem;                                                            \
   void* cf_signals[2];                                                        \
@@ -47,10 +47,10 @@
   char* realpath;                                                             \
   int realpath_len;                                                           \
   int cf_flags;                                                               \
-  void* cf_eventstream;                                                       \
+  void* cf_event;                                                             \
   uv_async_t* cf_cb;                                                          \
-  void* cf_events[2];                                                         \
-  uv_sem_t cf_sem;                                                            \
+  void* cf_member[2];                                                         \
+  uv_sem_t _cf_reserved;                                                      \
   uv_mutex_t cf_mutex;                                                        \
 
 #define UV_STREAM_PRIVATE_PLATFORM_FIELDS                                     \

src/unix/darwin.c

@@ -38,7 +38,7 @@
 
 
 int uv__platform_loop_init(uv_loop_t* loop, int default_loop) {
-  loop->cf_loop = NULL;
+  loop->cf_state = NULL;
 
   if (uv__kqueue_init(loop))
     return -errno;