systemd-analyze: filter dot output

Make "systemd-analyze dot" output only lines with units matching
given glob(7) patterns. Add --from-pattern and --to-pattern options.
Without any patterns all relationships are printed as before.

A relationship must match the follwing expression:

    (isempty(from) || from[0] || from[1] || .. || from[n]) &&
    (isempty(to) || to[0] || to[1] || .. || to[n]) &&
    (isempty(P) || P[0] || P[1] || ... || P[n])

where from[] and to[] are lists of patterns provided with subsequent
--from-pattern and --to-pattern respectively. P[] is a list of  additional
patterns provided after the "dot" subcommand.
man: be clearer that it's not OK to manipulate systemd's own cgroup hirearchy
analyze: add missing --help text entries
analyze: fix OOM handling + fix memory leak
analyze: various cleanups
cgroup: clean-ups
update TODO
journald: bring max coredump size in sync with max entry size