Increase maximal size of controller values

Increase maximal size of controller values

Maximal length of a controller value is determined by CG_VALUE_MAX,
which is equal to 100. That is not sufficient in some cases.

Add new constant CG_CONTROL_VALUE_MAX (to prevent breaking current API)
and set it to 4096, which is usually equal to the amount of bytes that
can be written to a sysctl file directly.

Add warning message about exceeding the limit while parsing
configuration file.

Signed-off-by: Nikola Forró <nforro@redhat.com>
Reviewed-by: Tom Hromatka <tom.hromatka@oracle.com>