WIP: Add nice_agent_set_port_exclusions()

Sets a list of ports that should not be used for allocating host candidates.

The provided ports is a comma-separated list of port definitions. A port definition can be a single port number (e.g. "12345"), or a port range (e.g. "11111-22222").

Example input: "1111,2222,5000-6000" -- This would tell libnice to avoid using ports 1111, 2222, and all ports between 5000 and 6000 (inclusive).

The rationale for this feature is that it adapts better to the needs that are common in typical cloud deployments, where a specific set of control ports should not be made accessible from the outside, with no reason whatsoever to prevent such access from all other ports in-between.

Just like nice_agent_set_port_range() defines a port "whitelist", the new nice_agent_set_port_exclusions() would define a port "blacklist".