Reworked libcgroup logging

Reworked libcgroup logging

Whole new logging with following goals is here:
 - more log levels.
 - allow applications to log somewhere else than stdout using their custom
   callback.
 - provide simple stdout logger for 'lazy' applications.

The logging is off by default, i.e. no message appears on stdout unless
application initializes the logging explicitly. But see following patches!

Internally, nothing changes, cgroup_dbg is still working. In addition,
cgroup_err, cgroup_warn and cgroup_info appeared. Description what message
should use which log level is in doxygen info, together with new public API
incl. simple example, I won't copy it here

Also the cgroup_log function is made public. I am not sure if it is the
right thing to do, but that's currently the simplest way how to use the
logging from our tools (which link only the public libcgroup API).

Changelog:
  - fixed typo in cgroup_log doxygen comment

Signed-off-by: Peter Schiffer <pschiffe@redhat.com>
Acked-by: Ivana Hutarova Varekova <varekova@redhat.com>