log: Swap contents of criu-log and log headers

The former one (criu-log.h) is now for service functions like logfile
management, pidfile and alike. It's only needed by files that set up
logging and for users that want specific functions like log_get_fd or
pr_quelled.

The latter (log.h) is purely for those who need pr_xxx helpers only.

Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>