Implement reading configuration file via VFS instead of sysctl.

Before this change the configuration data was readed only through the
sysctl interface, so you had to write all the configuration to /proc to
send it to the Tempesta FW.
Now the configuration may be located anywhere in VFS and you specify
only a path to it via net.tempesta.cfg.path.

Related to:
#18 - Tempesta FW configuration framework