Fix memory leak in stdout journal streams

Just as 'identifier' is strdup-ed and freed, we need to do the same for
unit_id.
bootchart: remove dead assignment
Trim TODO
kernel-install: add -h/--help
Help output spring cleaning

Use [brackets] only for optional elements.
Use <optional> in XML sources.
systemctl: simplify argument parsing
systemctl: fix 'is-enabled'
man: document 'is-enabled' output

https://bugzilla.redhat.com/show_bug.cgi?id=953077