Add isc_uv_export()/isc_uv_import() functions to libuv compatibility layer.

Add isc_uv_export()/isc_uv_import() functions to libuv compatibility layer.

These functions can be used to pass a uv handle between threads in a
safe manner. The other option is to use uv_pipe and pass the uv_handle
via IPC, which is way more complex.  uv_export() and uv_import() functions
existed in libuv at some point but were removed later. This code is
based on the original removed code.

The Windows version of the code uses two functions internal to libuv;
a patch for libuv is attached for exporting these functions.