windows/fs: wrap multi-statement macros in do..while block
windows/fs: make uv_fs_open() report EINVAL correctly

Before, when the user passed an invalid paramter to uv_fs_open, libuv
would detect this and call SET_REQ_RESULT to set the result value to -1.
SET_REQ_RESULT then stored whatever error code was returned by
GetLastError(), which would have no relationship to the actual problem,
and might as well be zero.
windows/fs: handle _open_osfhandle() failure correctly

Until now we assumed that _open_osfhandle() would set _doserrno on
failure. This assumption was very wrong in one obvious case, namely when
the CRT file descriptor table would fill up. In that case errno is set
to EMFILE, but GetLastError() returns zero - which makes sense because
it's not a win32 error but rather a CRT problem.