buildlet: don't leak file descriptors to buildlets

Also, add count of fds and goroutines to the coordinator's status
page.

Change-Id: I857e609623cfa280716d5d079180d0e4021d0bac
Reviewed-on: https://go-review.googlesource.com/27550
Reviewed-by: Quentin Smith <quentin@golang.org>
2 files changed