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.