Allow binding to any port.

This patch modifies the BindToAddress and DataBindToAddress
configuration options to allow binding to any port by specifying "*" as
the port number.

This makes a lot of sense for the data port as it can result in somewhat
better connectivity when there are multiple tinc nodes behind a single
NAT, by making source ports less likely to be rewritten.

For the meta port it's probably not very useful, but is nonetheless made
possible for consistency.

Note that unfortunately, using "*" for both address and port is not
possible (even though it makes sense) because getaddrinfo() explicitly
disallows setting both node and service to NULL for some reason.