replace strncat (unsafe) with strlcat (safe)

[warrenburch]

Some code bases will not allow strncat anymore for safety concerns. Replace with strlcat, note when using strlcat you simply pass the full size of the buffer (you do not account for the NULL terminator, this is handled for you).

I tested this by forcing inclusion of features using ...

CFLAGS='-DHAVE_CPU_AFFINITY -DHAVE_FLOWLABEL -DHAVE_TCP_CONGESTION -DHAVE_SENDFILE -DHAVE_SO_MAX_PACING_RATE -DHAVE_SO_BINDTODEVICE -DHAVE_DONT_FRAGMENT -DHAVE_PTHREAD' ./configure

NOTE: I tried to include SCTP and SSL but both failed to compile for missing headers.

I tested the

TAILQ_FOREACH(t, &(test->server_output_list), textlineentries) {
strlcat(output, t->line, buflen+1);
printf("otuput line: %s\n", t->line);
}

section by forcing

    struct iperf_textline *line1 = malloc(sizeof(*line1));
    struct iperf_textline *line2 = malloc(sizeof(*line2));
    
    line1->line = strdup("First line\n");
    line2->line = strdup("Second line\n");
    
    TAILQ_INSERT_TAIL(&(test->server_output_list), line1, textlineentries);
    TAILQ_INSERT_TAIL(&(test->server_output_list), line2, textlineentries);

beforehand. Tested error path by forcing error in server, force the IEDAEMON error with errno=999. Error string was as expected.

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• Version of iperf3 (or development branch, such as master or
  3.1-STABLE) to which this pull request applies:

• Issues fixed (if any):

• Brief description of code changes (suitable for use as a commit message):