unix, windows: move uv_now() to uv-common.c
test: use c-style comments

Fixes a compilation problem on OS X caused by the use of c++-style
comments in test-osx-select.c.
unix: run expired timers in UV_RUN_ONCE mode

Before this commit, creating an event loop, starting a timer and
calling uv_run(UV_RUN_ONCE) blocked in uv_run() until the timer
expired - but didn't actually run the timer.