config: add support for CRIU_CONFIG_FILE

With this it is possible to point the environment variable
CRIU_CONFIG_FILE to a CRIU configuration file.

The order the configuration files are evaluated now is:

 1. global (/etc/criu/default.conf)
 2. user ($HOME/.criu/default.conf)
 3. CRIU_CONFIG_FILE
 4. --config FILENAME
 5. CLI

Signed-off-by: Adrian Reber <areber@redhat.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>