unix: set errno in sendfile emulation

The sendfile emulation in src/unix/fs.c polls the file descriptor for
write readiness. If POLLERR or POLLHUP is set, it bails out but doesn't
set errno (hence it doesn't report a useful error code). Rectify that.

Fixes #620.
build: turn on strict aliasing in release builds
build: fix misleading vcbuild.bat error message
windows: align uv_run code with unix
unix, windows: add uv_stop, stop running event loop
unix: make uv_stop work when embedded

* Make uv_stop() work when libuv is embedded in another event loop.

* Fix a small bug where loop->stop_flag was not reset when mode ==
  UV_RUN_ONCE or UV_RUN_NOWAIT. The next call to uv_run() would return
  immediately without doing any work.
windows: fix uv_stop in ONCE and NOWAIT modes

Same as the fix in 492efb9 but this time for uv-win.
test: don't rewind_cursor when using tap_output
windows: initialize stop_flag explicitly

The default loop lives in the bss section so it's zeroed on startup
but loops created with uv_loop_new() live on the heap and contain
random garbage. Initialize the stop_flag explicitly to avoid spurious
bugs.