feat(handheld): import a handful of customizations from SteamOS

Added the `memlock` config from Bazzite. SteamOS preloads `hid-nintendo` and `hid-playstation` to avoid a race condition when first connecting these controllers that causes Steam to fallback to evdev instead of hidraw. `20-steamos-customizations.conf` also imports the various kernel parameters that SteamOS applies such as setting the timeout for abandoned TCP connections or disabling splitlock mitigations.