Enable sendto for syslog logging to /dev/log in prelogin

We write to log from check_limits() and therefore we have to allow
sendto() for syslog otherwise sandbox will kill the child.

Index: vsftpd-3.0.2/seccompsandbox.c
===================================================================
--- vsftpd-3.0.2.orig/seccompsandbox.c
+++ vsftpd-3.0.2/seccompsandbox.c
@@ -388,6 +388,11 @@ seccomp_sandbox_setup_prelogin(const str
                        1, PF_FILE,
                        2, SOCK_DGRAM | SOCK_CLOEXEC,
                        3, 0);
+  // allow syslog logs from check_limits()
+  if (tunable_syslog_enable)
+  {
+    allow_nr_1_arg_match(__NR_sendto, 6, 0);
+  }
 
 }