init: ignore ENOENT from fewer builtins

Previously we were ignoring ENOENT from all builtins as
rootdir/init.rc has many legacy commands that we need to keep for
backwards compatibility, but are otherwise no longer relevant.

However, this wasn't catching actual issues, for example chown failing
due to not finding the user or group name.  This change therefore
reduces the scope of ignoring ENOENT to the only the extraneous errors
in builtins.

Test: boot CF and walleye without seeing errors from init.rc
Test: see errors from invalid users/groups in chown
Change-Id: Ia8e14fa2591e083cb1736c313a3e55515bc5d15e