vendor c/{buildah,common}: appendable containers.conf strings, Part 1

This change is the first step of integrating appendable string arrays
into containers.conf and starts with enabling the `Env`, `Mounts`, and
`Volumes` fields in the `[Containers]` table.

Both, Buildah and Podman, read (and sometimes write) the fields of the
`Config` struct at various places, so I decided to migrate the fields
step-by-step.  The ones in this change are most critical ones for
customers.  Once all string slices/arrays are migrated, the docs of
containers.conf will be updated.  The current changes are entirely
transparent to users.

Signed-off-by: Valentin Rothberg <vrothberg@redhat.com>

vendor/github.com/containers/buildah/chroot/seccomp_unsupported.go

@@ -9,6 +9,8 @@ import (
 	"github.com/opencontainers/runtime-spec/specs-go"
 )
 
+const seccompAvailable = false
+
 func setSeccomp(spec *specs.Spec) error {
 	if spec.Linux.Seccomp != nil {
 		return errors.New("configured a seccomp filter without seccomp support?")