daemon: check existence of config file

... even if rundir isn't specified.  No other changes in semantics.
Before this a typo in config path would pass silently.
5 jobs from check-config in 34 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Build
canceled #73054
docker linux amd64
build:linux:amd64

passed #18819
docker linux amd64
build:linux:amd64

00:00:36

 
  Test
passed #18821
docker linux amd64
deckard:linux:amd64

00:00:33

canceled #18822
docker linux amd64 allowed to fail
respdiff:linux:amd64

00:00:11

passed #18820
docker linux amd64
test:linux:amd64

00:00:21