Untangle locale-setting code

[d235j]

At the moment the locale-setting code (in lib/scopehal/Unit.h) uses the C API and does not appear thread-safe on Windows. Additionally, on macOS it seems to not correctly reset the locale on application exit.

Previously there were issues with file interchange due to locale (#494). The OS is supposed to set the application's locale to C on startup, but obviously something isn't right, so this needs to be thoroughly investigated to clean up the API calls and prevent further interchange issues.

[azonenberg]

Conjecture: GTK was doing something with locales in glscopeclient that might no longer be an issue now that we're not using GTK?