Add 8bit-version of get_process_cmdline() and use in cgroup-show.c

Add 8bit-version of get_process_cmdline() and use in cgroup-show.c

This restores show_pid_array() output in legacy locales on the console.
Only one call to get_process_cmdline() is changed, all others retain
utf8-only mode. This affects systemd-cgls, systemctl status, etc, when
working locally.

Calls to get_process_cmdline() that cross a process boundary always use
utf8. It's the callers responsibility to convert this to some encoding that
they use. This means that we always pass utf8 over the bus.