This keeps one central place for the XDG switch.  Specifically, this
disables the XDG listener on Apple, where XDG is not supported.

Changed listen_add_config_param() to use generic address parsing

Passing it by value is actually smaller (32 bit) than the rvalue
reference (64 bit pointer), and it ensures that the object is consumed
after the call returns, no matter how the methods are implemented.

Move code from config_parse_path().

Don't use global variables.

The new class Path only holds a string pointer without being
responsible for allocation/deallocation.  The FileSystem.hxx library
accepts Path arguments instead of AllocatedPath, to avoid forcing
callers to allocate another string object.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.