add tcp-syncnt URL param to adjust TCP_SYNCNT sockopt

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.

Example:
 examples/nfs-ls nfs://10.0.0.1/export?tcp-syncnt=1

Signed-off-by: Peter Lieven <pl@kamp.de>