od-unix: persist host settings in winuae.ini

od-unix: persist host settings in winuae.ini

The Unix port had no equivalent of the Windows registry/winuae.ini
store: the Qt Paths page reverted to hardcoded defaults on every start
(GitHub issue #2).

Port the ini-file mode of the od-win32 registry abstraction into
od-unix/registry.cpp on top of the shared ini.cpp, with the same
registry.h API. The store resolves to winuae.ini next to the executable
when present (portable mode, matching Windows), otherwise
$XDG_CONFIG_HOME/winuae/winuae.ini on Linux and
~/Library/Application Support/WinUAE/winuae.ini on macOS; the
WINUAE_INI environment variable overrides the location.

The Qt launcher loads the Paths page directories and flags from the
store and saves them when the dialog closes, through new host-setting
hooks on the provider struct. The core path fetchers consult the stored
values between per-config unix.*_path overrides and the built-in
defaults, so the configured directories also apply outside the GUI.
Settings flush on dialog close and at gui_exit.

Covered by winuae_unix_registry_test (value types, subtrees,
enumeration, deletion, and persistence across reopen).