bugfix: disabled dtrace in the libuv build because it caused issues when recent versions of systemtap is installed.
Make -6 completely optional by using Perl require instead of use.

If the system does not have IO::Socket::INET6 installed then trying
to get an IPv4 free port will fail. By switching to require that
module will only be loaded if the -6 option is used.
Remove dependence on free-port.pl

This makes the test suite a little less flexible and fixes the test
port to 30498, but it prevents people needing a correctly set up
Perl on their system.
Tests for ECDSA signing