windows: fix compatibility with cygwin pipes

This makes libuv more tolerant to the properties of the pipes
that it can use without any issue. This is necessary because Cygwin
(and hence Mintty) opens STDIN without FILE_WRITE_ATTRIBUTES.
windows: count queued bytes even if request completed immediately

The send_queue_size/count needs to maintain its value until right before
the callback is called.
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
API documentation cleanup.