Rename os to opsys in PluginCore::setPlatform()

`os` is used as function argument name in `setPlatform()` and also in the `FBLOG_LOG_BODY` macro as local variable name (in the `do { ... } while(0)` scope). The latter `os` masks the outer scope function argument `os` in `FBLOG_INFO` so that the macro becomes invalid. This (rightly) confuses VS 2013.