windows: disable CRT debug handler on MinGW32

We cannot actually use it because the header is not present, so disable
it. On MinGW-w64 it does work though.
build: fix build on MinGW32
windows: map ERROR_INVALID_DRIVE to UV_ENOENT
doc: cleanup API documentation in uv.h
unix: try to write immediately in uv_udp_send
test: fix watcher_cross_stop

It's possible that recv_cb_called is bigger than the number of sockets,
because if all sends succeed the recv callback is called twice: once
with the actual data, and another time with 0.