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.

include/iscsi-private.h

@@ -89,6 +89,7 @@ struct iscsi_context {
 	int tcp_keepcnt;
 	int tcp_keepintvl;
 	int tcp_keepidle;
+	int tcp_syncnt;
 
 	int current_phase;
 	int next_phase;