core: allow changing TCP keepalive packet idle time using `tcp-keepalive` command-line option or config-file option.

This may be useful in certain scenarios. However, server load from keepalive
is increased 6-fold if code is hard-changed from 30 to 5. So, provide it as
an option instead, and use the previous value as a default (30).

Explanation from

015c064396

Kevin's middlecoin fix, CURL TCP keepalive constants lowered:

CURLOPT_TCP_KEEPIDLE from 45 to 5 and CURLOPT_TCP_KEEPINTVL from 30 to
5. Before it'd trigger a keepalive packet after 45 seconds of connection
idle time and then again every 30 seconds. Now it triggers a keepalive
packet after 5 seconds of connection idle time and then again every 5
seconds.

It makes the client more resilient against coin switching pools or just
pools with connection issues in general. It will however add a tiny bit
pressure to the pool server; but a TCP keepalive probe is only about
60-80 bytes, so I don't think it is an issue.

cgminer.c

@@ -135,6 +135,11 @@ bool opt_delaynet;
 bool opt_disable_pool;
 static bool no_work;
 bool opt_worktime;
+#if defined(HAVE_LIBCURL) && defined(CURL_HAS_KEEPALIVE)
+int opt_tcp_keepalive = 30;
+#else
+int opt_tcp_keepalive;
+#endif
 
 char *opt_kernel_path;
 char *cgminer_path;
@@ -1176,6 +1181,14 @@ static struct opt_table opt_config_table[] = {
 			opt_set_bool, &use_syslog,
 			"Use system log for output messages (default: standard error)"),
 #endif
+	OPT_WITH_ARG("--tcp-keepalive",
+		     set_int_0_to_9999, opt_show_intval, &opt_tcp_keepalive,
+#if defined(HAVE_LIBCURL) && defined(CURL_HAS_KEEPALIVE)
+		     "TCP keepalive packet idle time"
+#else
+		     opt_hidden
+#endif
+	),
 #ifdef HAVE_ADL
 	OPT_WITH_ARG("--temp-cutoff",
 		     set_temp_cutoff, opt_show_intval, &opt_cutofftemp,

miner.h

@@ -971,6 +971,7 @@ extern bool opt_delaynet;
 extern bool opt_restart;
 extern bool opt_worktime;
 extern int swork_id;
+extern int opt_tcp_keepalive;
 
 #if LOCK_TRACKING
 extern pthread_mutex_t lockstat_lock;

util.c

@@ -255,13 +255,11 @@ static void set_nettime(void)
 #if CURL_HAS_KEEPALIVE
 static void keep_curlalive(CURL *curl)
 {
-	const int tcp_keepidle = 45;
-	const int tcp_keepintvl = 30;
 	const long int keepalive = 1;
 
 	curl_easy_setopt(curl, CURLOPT_TCP_KEEPALIVE, keepalive);
-	curl_easy_setopt(curl, CURLOPT_TCP_KEEPIDLE, tcp_keepidle);
-	curl_easy_setopt(curl, CURLOPT_TCP_KEEPINTVL, tcp_keepintvl);
+	curl_easy_setopt(curl, CURLOPT_TCP_KEEPIDLE, opt_tcp_keepalive);
+	curl_easy_setopt(curl, CURLOPT_TCP_KEEPINTVL, opt_tcp_keepalive);
 }
 #else
 static void keep_curlalive(CURL *curl)