init: reset errno in do_start

do_start() ignores ENOENT intentionally to avoid logspam. It's
implemented in ErrorIgnoreEnoent. However, without resetting errno,
ErrorIgnoreEnoent will ignore unrelated errors from Service::Start() due
to the sticking errono set from other commands.

Bug: 296821716
Test: launch_cvd
Change-Id: I71d3113bdb69bdca82e2ff4f3a793301749f6c08