unix, windows: always update loop time

Fixes #846
windows: translate system errors in uv_spawn

We weren't always translating system errors for in uv_spawn.

This is half a fix for node's unit test
 test-child-process-cwd.js on win.
 The rest will go into node lib.
 It also fixes test-child-process-exec-error
windows: uv_spawn code refactor