zebra: deal with irdp compile warnings

zebra: deal with irdp compile warnings

* zebra/irdp.h
  * irdp_send_thread(): move prototype to common header file
  * irdp_advert_off(): idem
  * process_solicit(): idem
  * irdp_read_raw(): idem
  * send_packet(): idem
* zebra/irdp_interface.c
  * inet_2a(): move function to where it is used
  * in_cksum(): lib/checksum.h already provides prototype
  * irdp_send_thread(): prototype moved away
  * inet_2a(): idem
  * irdp_advert_off(): idem
  * b1, b2, b3, b4: get rid of global buffers for inet_2a()
  * if_group(): add local buffer for inet_2a()
  * if_add_group(): idem
  * if_drop_group(): idem
  * irdp_config_write(): idem
  * Adv_new(): make static
* zebra/irdp_main.c
  * irdp_read_raw(): prototype moved away
  * send_packet(): idem
  * in_cksum(): lib/checksum.h already provides prototype
  * inet_2a(): function moved to irdp_interface.c
* zebra/irdp_packet.c
  * b1, b2, b3, b4: get rid of global buffers for inet_2a()
  * in_cksum(): lib/checksum.h already provides prototype
  * process_solicit(): prototype moved away
  * irdp_read_raw(): fix uninitialized variable