libuv integration

This gets the libuv stuff plumbed in and working.

Currently it's only workable for some service thread, and there
is an isolated valgrind problem left

==28425== 128 bytes in 1 blocks are definitely lost in loss record 3 of 3
==28425==    at 0x4C28C50: malloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==28425==    by 0x4C2AB1E: realloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==28425==    by 0x58BBB27: maybe_resize (core.c:748)
==28425==    by 0x58BBB27: uv__io_start (core.c:787)
==28425==    by 0x58C1B80: uv__signal_loop_once_init (signal.c:225)
==28425==    by 0x58C1B80: uv_signal_init (signal.c:260)
==28425==    by 0x58BF7A6: uv_loop_init (loop.c:66)
==28425==    by 0x4157F5: lws_uv_initloop (libuv.c:89)
==28425==    by 0x405536: main (test-server-libuv.c:284)

libuv wants to sign off on all libuv 'handles' that will close, and
callback to do the close confirmation asynchronously.  The wsi close function
is adapted when libuv is in use to work with libuv accordingly and exit the uv
loop the number of remaining wsi is zero.

Signed-off-by: Andy Green <andy.green@linaro.org>