Test: fix instances.tcl restart_instance abort error.
Test: find_available_port: check that cluster port is free as well.

The function will only return ports that have also port+10000 free, so
that Redis Cluster instances can be executed at the returned port.
Fix Solaris compilation due to ctime_r() call.

Introduced in Redis 2.8.10 because of a change in Sentinel.
This closes issue #1837.