add tst_get_unused_port()

Returns unused port number for specified domain/type.

Signed-off-by: Jan Stancek <jstancek@redhat.com>
Acked-by: Cyril Hrubis <chrubis@suse.cz>
do not pick free/unused port by chance

Let the kernel pick free random port and do not rely on previously used
formula: (getpid() % 32768) + 11000 to provide unused port numbers.

If test only needs unused port number (without binding anything to it)
get it with tst_get_unused_port().

Signed-off-by: Jan Stancek <jstancek@redhat.com>
Acked-by: Cyril Hrubis <chrubis@suse.cz>