Full support for ff log file.

Full support for ff log file.

1. You can enable ff log file in config.ini.
2. These log will write to the specified log file:
  2.1 DPDK log after rte_eal_init.
  2.2 F-Stack lib log after ff_load_config.
  2.3 F-Stack FreeBSD log, include printf()/vlog(), and
  log() also modify from syslog(/var/log/message) to log file.
3. APP can use ff_log API set its own log file to instead
the default/config.ini's log file.