Reworked logging configuration

Reworked logging configuration

Debug log messages are now automatically compiled in libcgroup unless
explicitly disabled with ./configure --disable-debug.

Please note that the messages still must be enabled in runtime by setting
appropriate log level, e.g. export CGROUP_LOGLEVEL=DEBUG.

Changelog:
  - removed --disable-debug option of configure script, removing debug
    messages we would save 'amazing' 4kB, which is IMO not worth it.

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