lib: fix more warnings on *BSD

lib: fix more warnings on *BSD

* zebra/kernel_socket.c: include "rt.h" to provide the prototypes of
  kernel_init() and kernel_terminate();

* lib/prefix.h: remove the deprecation warning whenever ETHER_ADDR_LEN
  is used. isisd uses the ETHER_HDR_LEN constant which is defined in
  terms of ETHER_ADDR_LEN in the *BSD system headers. So, when building
  FRR on *BSD, we were getting several warnings because we were using
  ETHER_ADDR_LEN indirectly;

* lib/command_lex.l, lib/defun_lex.l: ignore other harmless warnings;

* lib/spf_backoff.c: cast 'tv->tv_usec' to 'long int' before printing.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>