linux: ignore fractional time in uv_uptime()

Before this commit, uv_uptime() returned the nanoseconds as the
fractional part of the uptime.  It's a bit inconsistent with the
output on other platforms because those only return whole seconds
(with the possible exception of Windows.)

This commit changes linux-core.c to only return whole seconds.
unix: fix SIGCHLD waitpid() race in process.c

Before this commit, multiple event loops raced with each other when a
SIGCHLD signal was received.  More concretely, it was possible for
event loop A to consume waitpid() events that should have been
delivered to event loop B.

This commit addresses that by doing a linear scan over the list of
child processes.  An O(n) scan is not terribly efficient but the
actual performance impact is not measurable in a benchmark that spawns
rounds of several thousands instances of /bin/false.  For the time
being, this patch will suffice; we can always revisit it later.

Fixes #887.
unix, windows: add uv_fs_event_start/stop functions

Make uv_fs_event behave like other handles, that is, it's inactive after
init, and it's active between start and stop.
.gitignore: add src/unix/uv-dtrace.h