windows: fix memory leaks in fs

Also clean up the code in various ways.

include/uv-private/uv-win.h

@@ -470,15 +470,16 @@ RB_HEAD(uv_timer_tree_s, uv_timer_s);
   int flags;                              \
   DWORD sys_errno_;                       \
   union {                                 \
-    wchar_t* pathw;                       \
-    int file;                             \
+    /* TODO: remove me in 0.9. */         \
+    WCHAR* pathw;                         \
+    int fd;                               \
   };                                      \
   union {                                 \
     struct {                              \
       int mode;                           \
-      wchar_t* new_pathw;                 \
+      WCHAR* new_pathw;                   \
       int file_flags;                     \
-      int file_out;                       \
+      int fd_out;                         \
       void* buf;                          \
       size_t length;                      \
       int64_t offset;                     \

include/uv.h

@@ -1410,8 +1410,8 @@ struct uv_fs_s {
   uv_fs_cb cb;
   ssize_t result;
   void* ptr;
-  char* path;
-  int errorno;
+  const char* path;
+  uv_err_code errorno;
   UV_FS_PRIVATE_FIELDS
 };