Add tmpfiles stanza for defining tmpfiles.d configurations.

This changes adds a new stanza tmpfiles that accepts list of paths to
tmpfiles.d configuration files. These will be executed before
pre-startup for the job. Note if any of the configurations returns
non-zero, the job will fail to start (by design).

BUG=chromium:1235329
TEST=FEATURES=test emerge-${BOARD} upstart

Change-Id: I825d13038465b2a3b44341e997e80984ae58a97e
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/upstart/+/3057689
Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
Tested-by: Allen Webb <allenwebb@google.com>
9 files changed