cgtop: major modernizations

cgtop: major modernizations

In preparation of the unified cgroup support, let's clean up cgtop:

a) rework time code to be based on "nsec_t" rather than "struct timespec"

b) Introduce long option --order= for selecting ordering

c) count number of processes only in the main hierarchy, don't bother
   with the controller hierarchies. We don't allow orthogonal
   hierarchies in systemd anymore, hence there's no point to check the
   other hierarchies.

d) Deal with non-monotonic cpuacct values (see #749)

e) When sorting groups, don't do prefix compare when ordering by number
   of tasks, since this is not accumulative for all children.

f) Actually make --cpu without parameter work

g) Don't output control characters when we get them as input.

Fixes #749.