Add cgroups_cpuacct plugin to collect CPU user/system time per cgroup

This is handy for computers using systemd, as systemd puts each service
into a separate cgroup.

Signed-off-by: Florian Forster <octo@collectd.org>
src/common.[ch]: read_file_contents: Use {s,}size_t rather than int.
cgroups_cpuacct plugin: Minor style fixes.

* Avoid mixed declarations.
* Use parse_value() rather than atoll().
* Comment static arguments to walk_directory().
* Return an error if the read() function can't locate the cgroup
  mount point.
cgroups_cpuacct plugin: Use type "cpu" to submit value lists.
cgroups_cpuacct plugin: Re-indent read_cpuacct_root().

The rest of the file is indented using tabs.
cgroups_cpuacct plugin: Read file line-by-line.

Use the field name from the file as type instance when dispatching values.