worker interactive mode: check stdin type

In particular, redirection from a file was leading to abort(),
so we provide an error message instead.
30 jobs for <span class="ref-name">memleak-uv-strdup</span> in 6 minutes and 36 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #204031
docker linux amd64
build:clang:linux:amd64

00:00:49

passed #204030
docker linux amd64
build:linux:amd64

00:00:50

passed #204032
docker linux amd64 allowed to fail
srpm

00:00:27

 
  Test
passed #204040
docker linux amd64
deckard:linux:amd64

00:01:31

passed #204048
privileged
distro:epel-7

00:02:48

passed #204047
privileged
distro:fedora-29

00:03:24

passed #204039
docker
doc

00:00:51

passed #204037
dind
docker:build

00:03:03

passed #204038
docker linux amd64
installcheck:linux:amd64

00:01:09

passed #204041
docker linux amd64
installcheck:valgrind:linux:amd64

00:02:43

passed #204034
docker
lint:c

00:02:43

passed #204035
docker
lint:clang-scan-build

00:00:57

passed #204033
docker
lint:lua

00:00:38

passed #204042
docker
osx:build

00:03:43

passed #204044
docker linux amd64
pytests:lint

00:00:36

passed #204045
docker linux amd64
pytests:run

00:01:51

passed #204046
docker linux amd64
respdiff:iter:udp:linux:amd64

00:02:20

passed #204036
docker linux amd64
test:linux:amd64

00:01:07

passed #204043
docker linux amd64
test:linux:amd64:valgrind

00:01:21

 
  Extended
canceled #204049
docker linux amd64
pytests:extended

00:01:48

canceled #204050
respdiff
respdiff:fwd-tls6-kresd.udp6

canceled #204051
respdiff
respdiff:fwd-udp6-kresd.udp6

canceled #204055
respdiff
respdiff:fwd-udp6-unbound.tcp6

canceled #204056
respdiff
respdiff:fwd-udp6-unbound.tls6

canceled #204054
respdiff
respdiff:fwd-udp6-unbound.udp6

canceled #204053
respdiff
respdiff:iter.tls6

canceled #204052
respdiff
respdiff:iter.udp6

canceled #204057
respdiff
resperf:fwd-tls6.udp-asan

00:01:53

canceled #204058
respdiff
resperf:fwd-udp6.udp-asan

00:01:53

canceled #204059
respdiff
resperf:iter.udp-asan

00:01:53