util: add functions getting proc cwd and root

/proc/[pid]/cwd and /proc/[pid]/root are symliks to corresponding
directories

The added functions returns values of that symlinks.
build: avoid trailing whitespace in feature string
sd-bus: create clean error when a property Set() call with incorrect signature is passed in
busctl: use canned error message on parse failure
busctl: when --address is specified do not assume we connect to a full bus
busctl: show property values in "introspect" output, add "set-property" command, and support both a terse and a verbose output format
tmpfiles.d: Create /var/lib/containers

Create /var/lib/containers so that it exists with an appropriate mode. We want
0700 by default so that users on the host aren't able to call suid root
binaries in the container. This becomes a security issue if a user can enter a
container as root, create a suid root binary, and call that from the host.
(This assumes that containers are caged by mandatory access control or are
started as user).