[BREAKING CHANGE] Hide source address when tui-privacy-max-ttl is set

[fujiapple852]

Currently, The tui-privacy-max-ttl flag can be used tp set the maximum ttl of the hop which will be hidden. Setting this to a value of zero (which is also the default) means that no hops will be hidden. Note that ttl are indexed from 1. The source of the trace, as shown in the Target line of the header, is always shown.

This change repurposes the zero value to indicate that the source of the trace, as shown in the Target line of the header, should be hidden. Values of 1.. are interpreted the same way as they are today.

Consequently, the default value has been changed to be none. Therefore, if not explicitly set (via command line argument or in the configuration file), nothing is hidden by default. If explicitly set to zero (the previous default) then the source of the trace will be hidden, and therefore this is a breaking change.