### Purpose
When generating a new `config.xml` file with default options, the GUI
address is populated with a hard-coded default value of
`127.0.0.1:8384`, except for a random free port if that default one is
occupied. This is independent from the GUI configuration default address
defined in the protobuf description. More importantly, it ignores any
`STGUIADDRESS` override given via environment variable or command-line
option, thus probing for the default port instead of the one specified
via override.
The `ProbeFreePorts()` function now respects the override, by reading
the `GUIConfiguration.Address()` method instead of using hard-coded
defaults.
When not calling `ProbeFreePorts()`, the override should still be
persisted rather than the default address. This happens only when
generating a fresh default `config.xml`, never on an existing one.
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