Add iscsi_set_tcp_syncnt function

This patch adds support for setting TCP_SYNCNT to overwrite
the system default values. This allows indirect support
for a configurable connect timeout.

Linux uses a exponential backoff for SYN retries starting
with 1 second.

This means for a value n for TCP_SYNCNT, the connect will
effectively timeout after 2^(n+1)-1 seconds.

lib/connect.c

@@ -238,6 +238,7 @@ try_again:
 	iscsi->tcp_keepidle = old_iscsi->tcp_keepidle;
 	iscsi->tcp_keepcnt = old_iscsi->tcp_keepcnt;
 	iscsi->tcp_keepintvl = old_iscsi->tcp_keepintvl;
+	iscsi->tcp_syncnt = old_iscsi->tcp_syncnt;
 
 	if (iscsi_full_connect_sync(iscsi, iscsi->portal, iscsi->lun) != 0) {
 		iscsi_destroy_context(iscsi);