systemctl,loginctl,cgls: do not ellipsize cgroup members when --full is specified

New file output.h with output flags and modes.

--full parameter also for cgls and loginctl.

Include 'all' parameter in flags (show_cgroup_by_path, show_cgroup,
show_cgroup_and_extra, show_cgroup_and_extra_by_spec).

get_process_cmdline with max_length == 0 will not ellipsize output.

Replace LINE_MAX with 0 in some calls of get_process_cmdline.

[zj: Default to --full when under pager for clgs.
     Drop '-f' since it wasn't documented and didn't actually work.
     Reindent a bit.
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