[server] Ignore pcap non-blocking setting in --pcap-file mode

When setting --pcap-file mode from the command line some versions of libpcap
do not appear to allow non-blocking mode to be set and throw the following
error:

[*] Error setting pcap nonblocking to 0:

This commit ignores the non-blocking setting in --pcap-file mode.

server/pcap_capture.c

@@ -171,7 +171,8 @@ pcap_capture(fko_srv_options_t *opts)
      *       to actually use this mode (which when set on a FreeBSD
      *       system, it silently breaks the packet capture).
     */
-    if((pcap_setnonblock(pcap, DEF_PCAP_NONBLOCK, errstr)) == -1)
+    if((pcap_file_mode == 0)
+            && (pcap_setnonblock(pcap, DEF_PCAP_NONBLOCK, errstr)) == -1)
     {
         log_msg(LOG_ERR, "[*] Error setting pcap nonblocking to %i: %s",
             0, errstr