[Fix] Use non-blocking mode for systemd sockets

Issue: #430
Reported by: @Twilek-de

rspamd.service

 After=nss-lookup.target
 
 [Service]
+Type=forking
+NonBlocking=true
 ExecStart=/usr/bin/rspamd -c /etc/rspamd/rspamd.conf -f
 User=_rspamd
 RuntimeDirectory=rspamd

src/rspamd.c

 				return NULL;
 			}
 			flags = fcntl (sock, F_GETFD);
+
 			if (flags != -1) {
 				(void)fcntl (sock, F_SETFD, flags | FD_CLOEXEC);
 			}
+
+			rspamd_socket_nonblocking (sock);
+
 			result = g_list_prepend (result, GINT_TO_POINTER (sock));
 		}
 		else if (num_passed <= number) {